‘Water Sensitive Cities’: Planning and Evaluation of Its Theoretical Application in a Mexican City with High Hydric Stress

Abstract

Cities can be viewed as a source of solutions to environmental problems. The Water-Sensitive Cities (WSC) model is part of the solution when trying to transform the current negative relationship between water and cities, since it has remained at the conceptual level, without evaluating the implications of its application in cities from developing countries. The primary aim of the article is to analyse a potential scenario under the WSC model, seeking an alternative solution to the problems of water availability and supply in cities in semi-arid environments with prominent levels of water stress from the Global South. We selected Aguascalientes City, Mexico, as a case study, and through geospatial analysis, it was possible to generate and assess urban planning strategies for stormwater harvesting and alternative sustainable infrastructure for urban and peri-urban areas. The results show that this analysis would imply a considerable reduction in groundwater extracted for urban consumption in the city, reducing local water stress; however, the various political and social implications intrinsic to the implementation of this model should be considered, as they may represent an obstacle to its execution.

1. Introduction

Water is an issue of central interest in the development of the world’s population. Its importance lies in its low global availability for human consumption and its uneven spatial distribution [1]. In fact, in the last two decades, water global consumption has doubled, and the demand is expected to increase by 20 or 30% by 2050 [2]. In this scenario, urban agglomerations have two facets. On one hand, they represent problems, as they are important causes of consumption, waste generation, overexploitation of aquifers, agglomeration of waste, soil contamination, and water pollution in general [3,4]. On the other hand, cities are viewed as a source of solutions to environmental problems and an alternative to achieve global sustainability [5,6,7,8]. The WSC model is part of these solutions, because it offers the possibility of redefining the relationship between the city and water through the combination of grey and green infrastructure [9], Water-Sensitive Urban Design, and integrated urban water management, along with water-oriented social systems, governance, and social engagement [10,11].

The Water-Sensitive City (WSC) model is a set of outlines and measurements that contribute to environmental repair and protection, water supply security, water quality, and economic sustainability, through Water-Sensitive Urban Design [12]. On the other hand, water stress occurs when the demand for extracted water exceeds the available volume during a given period or when inadequate quality restricts its use [13]. Thereby, the application of the WSC model in cities located in areas with water stress could be helpful to lighten it or even overcome it, because it could contribute to the designing of urban and environmental policies. The WSC model has been adopted mainly in developed countries, mostly in Australian cities [14], and on fewer occasions in European [15,16] and Asian cities [17]. In these cases, the conceptualization and materialization of the model has been conducted only in some specific sections or areas of the city. Therefore, we wonder about the possible potential of the model as an alternative to reduce water stress in a city when applied at an urban scale.

The primary aim of the article is to evaluate the water balance of the city of Aguascalientes after the implementation of rainwater conservation measures, through the application of a potential scenario in which the WSC model is adopted, in an attempt to present an alternative solution to availability problems and water supply in cities located in semi-arid environments from the Global South and with high levels of water stress. We selected Aguascalientes City, Mexico, as a case study because it has the characteristics previously described, therefore making it crucial to find alternative solutions for water scarcity and increasing demand, focusing on the formulation of urban planning strategies for stormwater harvesting and alternative sustainable infrastructure for water management in urban and peri-urban areas. Through micro-basins geospatial analysis, areas with high rain catchment potential were identified and differentiated through a zoning process. Green and grey infrastructure are proposed in each case, from which different water collection scenarios are estimated with a prospective vision. This makes it possible to generate and assess urban planning strategies for stormwater harvesting and alternative sustainable infrastructure for urban areas.

This work contributes to the discussion about the potential that the application of the WSC model at an urban scale could have on the pressure levels of aquifers in cities with high water stress in the Global South. The results obtained show that, at least in theoretical terms, the volume of water intake derived from the application of the infrastructure that constitutes such a model significantly reduces the levels of water extraction from the subsoil and decreases the water stress of the city. However, despite its potential, the application of the model has at least two limitations: on the one hand, its financial cost (construction and maintenance), and, on the other, the social and political costs involved in implementing the model. The results obtained allow us to glimpse possibilities for the design of urban policies aimed at the configuration of urban space as an alternative response to current environmental challenges.

This article is structured in four parts, in addition to this introduction. The first presents the conceptual framework within which the characteristics of the Water-Sensitive City model and its relationship with urban planning are addressed. In the second, the methodological strategy conducted to prepare the study is detailed, whose main characteristic is to differentiate urban and peri-urban micro-basins. In the third, the empirical results of the work are described, and the water scope that the proposal could have in different scenarios is studied. In the fourth, some final reflections are formulated, with particular emphasis on the implications that the application of the WSC model would have in socio-political terms. Finally, the bibliography that was used to prepare the article is presented.

2. Conceptual Framework: Water Sensitivity in Urban Areas

2.1. Is WSC Model an Alternative Solution to Water Stress in Cities?

The WSC concept was introduced for the first time in 2004, as one of the objectives included in the Australian Commonwealth’s National Water Initiative in the chapter corresponding to urban water management reforms. Although both the concept and its scope were not fully defined, recent studies and initiatives suggest that a WSC might help ensure environmental repair and protection, supply security, public health, and economic sustainability, through Water-Sensitive Urban Design [18,19].

The emergence of Water-Sensitive Urban Design (WSUD) in Australia was part of a broader international movement towards the integrated concept of land and water management. It was first introduced in several publications exploring concepts and possible structural (infrastructure) and non-structural (policies and regulations) practices concerning urban water resources management in the early 1990s, and the concept has been evolving since then [20]. In general, all different conceptualizations refer to the integration of urban planning with the management, protection, and conservation of the urban water cycle with the built environment through urban planning and design, which guarantees that water management is sensitive to natural hydrological and ecological processes [12,21,22,23,24]. This integration seeks the creation of places where all the elements of the water cycle and their interactions are considered concurrently to achieve a result that sustains a healthy natural environment while meeting human needs; this includes, for example, affordable water and good service, flood pathway integration, local resource management, wastewater reduction and treatment, rainwater and surface water runoff recycling, and habitat enhancement.

A WSC is positioned as a water management solution that seeks to integrate physical infrastructure along with social systems, governance, and social commitment, intending to create a city in which the connections that people have with their infrastructure and water services improve their value and quality of life, always looking forward into the future [11,25]. To achieve the goals of a WSC model, it is necessary to develop a novel approach that integrates spaces dedicated to water, strengthen an urban culture that values and protects the resource, ensure that the cultural transformation makes an impact on the different stakeholders, and promote the implementation of sustainable technological alternatives [20,26]. The first step is to transform the current conception of stormwater runoff as a waste, understand it as a resource, and recognize the city as part of the solution.

Commonly, the application of this model implies the need to: (i) increase the possibility of providing alternative sources of water supply, reducing in consequence the existing problem of water overexploitation; (ii) reduce the occurrence of some of the causes that favour the worsening of such problem; (iii) construct innovative and environmentally compatible infrastructure; (iv) construct a process of social improvement by changing patterns of use and exploitation of resources in a responsible manner, (v) combine and implement non-structural measures, which together would make it possible to achieve permanent improvement in the long term and would guarantee the proper implementation of policies and programs aimed at a sustainable use of water.

2.2. WSC Model and Its Link to Urban Planning

The WSC model uses improved urban planning to reuse stormwater, preventing it from reaching natural water bodies by mimicking the natural water cycle as much as possible. Under this scheme, rainwater harvesting involves collecting, treating, storing, and using stormwater runoff from drains and roofs in urban areas [27].

Water-Sensitive principles aim to protect natural systems according to the following elements: protecting and enhancing natural aquatic systems within urban developments; integrating stormwater treatment into the landscape using stormwater in multiple-use corridors that maximize visual and recreational value; protecting the quality of the water; reducing runoff and peak flows from urban developments through local detention measures and minimizing impervious areas; and lastly, adding value and minimizing development costs by reducing the cost of sanitation infrastructure [22].

These principles require an integrative approach to land and water planning at all levels of the urban development process. According to Bracken [28], it is considered as a problem-solving process, where answers and proposals can be found through a systematized process that allows defining strategies and goals to be met, for which it is necessary to understand the nature of the problem and its possible solutions.

A WSC entails advancing through six progressive levels of attention to the water in a city [10,19]. The first three stages describe the evolution of the water system to provide essential services such as safe access to drinking water (“Water Supply City”), protection of public health (“Sewered City”), and protection against floods (“Drained city”); these are followed by “Waterways City”, “Water Cycle City”, and, ultimately, WSC. The latter offers a range of services, including recreational services (for green spaces and enhancing the effects of the city’s heat islands) and environmental protection, reliable water services with limited resources (including access to non-climate-dependent sources, such as aquifers), intergenerational equity, and resistance to climate change. The transition to a WSC requires significant changes in its structures, cultures, and practices that support the planning, design, and management of water systems and urban developments [25].

In a complementary way, it is proposed that the traditional concept of the urban water cycle, which linearly incorporates supply and sanitation services, should be replaced by a more comprehensive and systemic vision, where water is linked to urban planning and development in addition to sustainability policies. This innovative approach is called Water-Sensitive Urban Development, in which existing water supply and distribution systems can be transformed to achieve integrated management of the urban water system in new models of sustainable urban development [29]. This model promotes an approach to urban development that adapts to the natural characteristics of places, while protecting natural ecosystems and optimizing the use of water as a resource.

In this way, the model sets up the bases to select different saving strategies and implementations of technologies for the comprehensive analysis of the urban water system. According to the consulted bibliography [30,31,32], the best strategies are those that make joint use of structural and non-structural measures.

Although there are different applied examples of the WSC model in Australia and Europe, they have been developed to address specific needs or problems of a specific area of the city or small populations. However, this academic proposal differs in tackling the local water problem, seeking benefits for the entire population, and considering the city as a whole. Once the results and the possibilities offered by this project are reviewed, it can be stablished in terms of hypotheses that the WSC model is a practical and efficient alternative to counteract water stress, but it should be part of a long-term project that allows not only the development of infrastructure but also the social and political changes that are essential to obtain the proposed goals. In the next section, we present the methodological elements to develop the central objective of this paper.

3. Methodology

3.1. Description of the Study Area

The study area is located in the state of Aguascalientes in central Mexico, in a region with a semi-arid climate, limited surface water reservoirs, and important water stress levels [33] due to heavy groundwater pumping, associated with a growing population and high water-consuming economic activities. The increase in water consumption in Aguascalientes has resulted in prolonged and severe pressure on water resources, with a deficit of −126.4 hm³ per year, thus placing local aquifers in a due to overexploitation and high water stress levels since 2006, a situation that worsens given an estimated loss from leaks of around 55 and 60% of extracted water [34]. It is expected that urban development, population growth, and water consumption will continue their trend, despite the safety of aquifers (

Table 1. City of Aguascalientes. Current situation and forecast.

	2018	2040 Forecast
Population (millions of people)	0.85	1.13
Total water consumption (millions of m3)	57.85	77.94
Annual water harvesting (dams) (millions of m3)	5.38	5.38
Consumption differential (millions of m3)	-	+20.09

Source: Data obtained from CONAPO, CCAPAMA, CONAGUA, and own calculations [35].

). With an expected population of 1.13 million people by 2040, water consumption would be greater than 20 million cubic meters, additional to what was extracted in 2018.

An alternative is raised to reduce groundwater extraction, relying on government information that suggests the city of Aguascalientes might have the right conditions to implement rainwater harvesting strategies that may add to a solution for water stress.

Aguascalientes City is located in the valley with the same name. Prior to urban development, there were 10 streams flowing in an east–west direction toward the San Pedro River, which represents the main current of the hydrological network of the region. Currently, most of these runoffs have lost their ecological function, since they have been piped or eliminated. However, the topography has not been substantially modified and the runoff features remain unaltered, giving place to the different micro-basins that the municipal offices have identified and delimited. Fourteen of these micro-basins have been selected and included in this analysis, taking as a reference those that spatially coincide with the urban area (Figure 1).

The scope of the study is determined by 14 micro-basins (

Table 2. Micro-basins linked to the city of Aguascalientes.

	Name	Area (ha)	%
1	Calvillito-Parga	10,353.75	13.9
2	Chicalote	3636.93	4.9
3	La Hierbabuena	6306.28	8.5
4	El Cedazo-San Antonio	6984.95	9.4
5	El Molino	1364.79	1.8
6	La Hacienda-San Nicolás	3309.18	4.5
7	Las Trancas-Morcinique	2943.29	4.0
8	Los Gringos-Los Arellano	2528.93	3.4
9	Don Pascual	1624.79	2.2
10	Las Trancas-Cueva del Tecolote	1420.25	1.9
11	San Pedro-San Francisco	4637.12	6.2
12	Salto de Montoro	10,601.27	14.3
13	La Escondida-Palo Seco	8958.90	12.1
14	San Francisco-Cobos-Paso Hondo	9671.37	13.0
	Total	74,342.68	100

Source: Data obtained from own geospatial calculations based on geospatial information generated by IMPLAN [36].

) associated with seasonal streams that run across the capital city, which covers an area of 74,341.79 hectares (Figure 1). The combination of the natural environment characteristics and their dynamics with the urban space and the population defines the territory’s potential for applying rainwater capture and storage strategies for its later distribution and use. The analysis of these factors determined the areas with high flooding potential (storage), those with the highest rainfall (capture), and those where water is consumed within the urban area (demand). The results show the priority areas to analyse, both at the urban and peri-urban level. Even though the delimitation of urban limits is based on those stablished in the PDUCA 2040, the model is not attached to the policies, guidelines, or strategies proposed in it. The above is due to several reasons: (a) the PDUCA does not contemplate guidelines or proposals for water management as part of urban planning strategies (neither this nor other municipal planning instruments), (b) the PDUCA covers only the urban area of the city of Aguascalientes, not the rest of the municipal territory (where the peri-urban proposals are raised). However, the urbanization limits are taken into account, since they coincide with the temporal scope and provide a spatial reference with respect to the area that could be used for the proposal of intra-urban strategies.

3.2. Materials and Methods

The project has an eminently urban planning approach, based on the desired future-oriented planning model, which seeks transformations in the social system through the redefinition of traditional values, norms, and long-term goals [28]. It includes the proposal of an alternative rainwater catchment and water supply system based on the principles of the urban-spatial WSC model. The results to be obtained are expected in the short, medium, and long term, mainly due to the social changes that the Water-Sensitive Urban Design model requires.

Based on [37], three elements with specific variables were analysed to design the rainwater harvesting system: (a) ‘Catchment Area’, for which elements of hydrology, topography, slope, soil texture, vegetation cover, and land use were analysed; (b) ‘Rainfall’, for which average monthly rainfall and maximum runoff flow were analysed; and (c) ‘Water Demand’, considering population, consumption, extraction, and distribution. Statistics were obtained from government entities such as the Municipal Planning Institute of Aguascalientes (IMPLAN), Citizen Commission of Potable Water and Sewerage of the Municipality of Aguascalientes (CCAPAMA), National Population Council (CONAPO), National Water Commission (CONAGUA), and National Institute of Statistic and Geography (INEGI).

The physical conditions of the catchment surface are those defined by the variables of the natural environment: hydrography, topography and slope, soil texture, vegetation cover, and land use. The hydrography of the study area includes ten seasonal streams and the San Pedro River (the main river in the region), which make up the natural drainage system of the study area, while the trend and magnitude of the rainfall were analysed by considering time records. With the rainfall data, the maximum flow of runoff was determined to identify those micro-basins that present the best natural conditions to implement rain catchment strategies because they present conditions such as floodplains.

To analyse the city’s water demand, population behaviour, and the latter’s relationship with water consumption, an evaluation based on the allocation per person assigned by type of lot as stipulated by the authorities was performed. The areas with the highest groundwater extraction were found based on their relationship with the local population density, and finally, an analysis of the city’s supply and sanitation systems was completed.

3.3. Analysis Strategy

The analysis was conducted by taking the micro-basins as the basic units of management and intervention where production can be managed and visualized, obtaining measurable indicators of sustainability [38]. To process geospatial information, the Analytical Hierarchical Process (AHP) was applied, which is based on the mathematics of map algebra implemented in Geographic Information Systems. The aim of implementing this analysis technique consisted, firstly, in finding the best areas for rainwater catchment according to their flooding potential; secondly, in calculating the runoff flows and the location of the micro-basins with the highest rates; and finally, the impact and pressure exerted by demographic variations and the demand for drinking water.

The data about the catchment area, rainfall, and water demand analysis were harmonized to obtain a zoning as a result of the raster calculator geoprocess carried out with GIS. Peri-urban and intra-urban zoning of intervention area (Figure 2) was obtained, and areas correspond to generally weighted zoning determined by the catchment capacity of each micro-basin according to the aforementioned variables. Through the analysis of geospatial and statistical information in a GIS of the three areas of analysis, it was possible to identify the most favourable areas within each micro-basin for the implementation of rainwater catchment and storage strategies based on the runoff flow and the available space to develop new infrastructure and to set up the approaches that would conform to the alternative supply system for the study area and maximize the benefits within the usable surface.

The zoning was carried out in two spatial areas: (1) peri-urban (hatched coloured areas in Figure 2), due to its high availability of space to develop large-scale strategies, and (2) intra-urban (solid-coloured areas in Figure 2), because of the limited availability of spaces to be developed and the relative difficulty of retrofitting pre-existing infrastructure to the new proposed system. The areas in blue and green tones correspond to those that present ideal conditions for larger catchment areas, while those in warm tones were considered for smaller infrastructure alternatives based on their feasibility of reconversion.

Each infrastructure proposal was made according to Lara [39] in order to favour either (a) retention by capturing rainwater from its point of origin, facilitating the treatment processes in the site through retention, sedimentation, and infiltration systems; (b) the protection and maintenance of natural conditions, seeking the preservation of the elements present in the existing natural drainage network (native plant species, watercourses, etc.); (c) the reduction in runoff by further reducing the impervious surface and allowing, in consequence, drainage to areas that promote retention and infiltration processes; or (d) the implementation of treatment chains for the elimination of pollutants. The application of a WSC model is proposed through the induced modification in the patterns of groundwater consumption. Since the problem is approached from an urban planning perspective, urban design or engineering proposals are excluded, which, although important, are not part of the scope of analysis and application of urban planning.

The potential consumption volume was obtained considering several factors, which include (a) the historical consumption records in the period from 1990 to 2018, (b) the consumption trend that was obtained, taking as reference the population growth projections for the city of Aguascalientes, (c) the consumption assignation per standard lot according to CCAPAMA classification, and (d) the maximum expected design rain catchment volume. The equation to obtain the potential consumption goes as follows:

$$\mathrm{PC}=\mathrm{CT}-\mathrm{RC}$$

(1)

where: PC = potential consumption; CT = consumption trend; RC = rainwater catchment.

To find the capacity of a bioretention element, the following information was needed: (a) the area and depth of the structure, (b) the drainage area that contributes to the average annual rainfall, and (c) the expected percentage of retention. With such data, the potential Total Runoff Retention (TTR) was calculated through bioretention and/or infiltration practices [40]. Thus, the calculation to obtain such retention is conducted according to the following equation:

TTR = [PP · (AE + AD) · % RE]

(2)

where: TTR = total runoff retention; PP = mean annual rainfall; AE = surface that will be occupied by the infrastructure; AD = drained area; % RE = expected retention percentage.

Without considering the drained area in the previous equation, the total amount of runoff from the installation of permeable surfaces can be calculated. Unlike bioretention and infiltration calculations, the percentage of rainfall that this alternative can obtain depends only on the available square meters and the maximum retention times set up for a specific site. In the next section, we present the empirical results of the previously presented methodological elements.

4. Results

4.1. Contextualization of the Problem

The study area has a rainwater catchment and storage infrastructure made up of three dams and seventeen berms used mainly for agricultural use (41.9% for irrigation, 9% for watering holes) and for recreational use (48.7%) [41]. Together, they have an approximate catchment and storage capacity of 5.4 million m³, which would be sufficient to satisfy 10% of the demand generated by the current city population. This infrastructure is concentrated, although not equally distributed, in nine of the fourteen micro-basins analysed, with the three largest, these being the Calvillito-Parga, Los Gringos-Los Arellano, and El Cedazo-San Antonio watersheds, comprising 74.7% of the total rainwater catchment and storage capacity in the study area.

Since the city lacks independent rainwater drainage that captures runoff separately from sanitary drainage, its later use becomes impossible; however, there is rainwater infrastructure in specific areas of the urban structure that has the purpose of capturing and directing runoff to the sanitary drainage for flood prevention, but not for later use whatsoever. Despite having infrastructure for the collection, storage, and treatment of wastewater in the study area, the stored water is not currently used for urban use except for the irrigation of green areas from the treatment plants. The destination of rainwater for urban use would imply a reduction in the extraction of groundwater during the rainy season, which would also result in an energy saving due to the decrease in the use time of the city’s pumps and the consequent decrease in the extraction.

4.2. Application of the WSC Model to Aguascalientes City: Structural Mechanisms

To achieve a WSC status, it is necessary to meet goals at distinct stages, which will set up the foundation to approach water sensitivity in the long term. Within Urban Water Management Transitions Framework it can be said that Aguascalientes City is still in the second phase, Sewered City. The implementation of rainwater catchment systems through the application of green infrastructure (GI) in the study area would allow advancing to the next stage of transition, the Drained City, which is characterized by using rain runoff, in addition to reducing the risk of flooding. Future urban water infrastructure, in a WSC, would capture and reuse an integrated mix of water sources (including stormwater, wastewater, and greywater). The management of all parts of the hydraulic network, including water supply catchments, wastewater, and demand management, is important to advance the goals of the WSC [42].

With all the above in mind, taking into consideration the intra-urban and peri-urban zoning obtained from the conjunction of the natural and socio-demographic conditions of the study area, a proposal for a green infrastructure program is shown to capture the greatest amount of rainwater, both at the origin of the runoff and within the urban structure, considering the treatment and distribution needed to build an alternative supply system to supply the city of Aguascalientes. Peri-urban strategies are applied in areas with remnants of natural vegetation found in the hilly areas in the east and south of the study area and the agricultural areas. Intraurban strategies focus on streets, urban streams or canals, recreational spaces, and abandoned land within the urbanized area.

The implementation of different elements of green infrastructure under the WSC model will allow the creation of a network of nodes and corridors made up of different strategies in the territory. This network, instead, will allow conserving both the ecological processes and the natural quality of the landscape and its elements. Furthermore, it will allow the generation of an alternative catchment, treatment, and distribution network that will contribute to decreasing the dependence on groundwater.

4.2.1. Peri-Urban Strategies

The peri-urban area has a large extension of usable land due to its lack of urbanized areas, which makes it ideal for implementing large rain catchment facilities, and, although they are located out of the urban limits of the PDUCA 2040, they are part of the municipal territory. Considering the peri-urban zoning map (Figure 2), the catchment strategies would focus on San Francisco-Cobos-Paso Blanco, Calvillito-Parga, Salto de Montoro, and La Escondida-Palo Seco micro-basins. New areas will naturally function as seasonal rainwater reservoirs, drawn on by and linked to larger reservoirs currently available in the study area (existing dams and berms), hence increasing the catchment infrastructure, re-enabling watercourses, and allowing natural treatment trains to be implemented, thus ensuring the accumulation of adequate water quality for later use in the urban area.

The strategies proposed for the peri-urban area include techniques with a large rain catchment capacity, such as constructed wetlands, detention tanks, and retention ponds, all of which can receive contributions from surfaces greater than 5 hectares; therefore, the need for large spaces without urbanization is critical (Figure 3). These strategies contribute to the maintenance or restoration of the natural ecological processes of the environment and maximize their permanence and long-term functionality.

Specifically, the proposal plans the creation of a system comprised of eight constructed wetlands and a retention pond linked to the existing catchment infrastructure in the study area to both increase storage capacity and enhance water treatment. The constructed wetland system would cover 354.43 hectares with a storage capacity of 23.27 million m³ of rainwater (

Table 3. Extent and capacity of proposed wetland systems.

No.	Name	Area (m2)	Catchment (Millions of m3) a	%
Parga Dam System		646,484.00	1.45	6.25
1	Parga 1	100,000.00	0.23	0.97
2	Parga 1a	100,000.00	0.23	0.97
3	Parga 2	40,201.00	0.09	0.39
4	Parga 3	6283.00	0.01	0.06
5	Las Grullas	400,000.00	0.90	3.87
La Hacienda-San Nicolás System		841,262.00	1.89	8.13
6	Arellano	841,262.00	1.89	8.13
Montoro System		1,456,542.00	18.58	79.82
7	Montoro	1,200,000.00	18.00	77.34
8	Montoro2	256,542.00	0.58	2.48
Hierbabuena System		600,000.00	1.35	5.80
9	Hierbabuena	600,000.00	1.35	5.80
	Total	3,544,288.00	23.27	100

^a The catchment capacity for each system was found based on the volumetric calculations of each catchment body, considering an average depth of 2.25 m in the case of the constructed wetlands. Fuente: Data obtained from geospatial calculations.

), equivalent to just over 40% of the annual water consumption registered in 2018. The implementation of the peri-urban proposal will lead to the creation of four micro-basin systems, and the construction of the catchment and storage structures will contribute to reducing the volume of rainwater runoff towards the urban structure in the study area.

The catchment and treatment systems added to the current storage capacity (assuming that the total collected water was destined for urban uses) would lead to a reduction in stress on the supply network by 50% (29.2 million m³) of that registered in 2018. Based on the projections made for the consumption of drinking water in the study area, it would be expected that by 2040, this would be even lower, approximately 36.8 million m³, than the volume reported in 2018, hence contributing to the reduction in the stress of the local aquifer. Although the consumption projections after the implementation of storage systems also show a linear growth trend, the volume of water projected for consumption is significantly lower and could even be improved by adding other mechanisms to water consumption for urban use.

4.2.2. Intra-Urban Strategies

The intra-urban infrastructure proposal does not focus on specific points or micro-basins, instead focusing on developing circuits and green infrastructure corridors that provide environmental and urban service to the whole city, therefore solving problems associated with the availability of water from alternative sources for urban use, in addition to those related to stormwater runoff, while fulfilling additional functions that provide a diverse variety of socio-environmental services.

Considering the lack of large spaces for the implementation of rainwater catchment strategies within the urban structure, the proposed strategies will have to meet specific goals associated with conflict zones due to flooding and barriers for runoff. It is suggested to implement a set of green infrastructure alternatives on important streets within Aguascalientes City, in such a way that together they comprise a series of circuits and corridors to maximize the socio-environmental services (Figure 4). This green infrastructure network is intended to be associated with the remaining natural urban streams within the city. Specific interventions are proposed for specific roads, some of which were built onto old watercourses, now piped, and are considered as conflict points due to flooding. The proposed roads to intervene were selected for being recurrent conflict points due to flooding in the rainy season, which makes them high-potential areas to develop rainwater catchment infrastructure.

Some of the applicable proposals include vegetated roofs, pervious surfaces, pervious surfaces with underground drainage, infiltration wells, bioretention basin, filtering or French drains, infiltration gardens, vegetated swales, and infiltration trenches. All the above can receive contributions from surfaces smaller than five hectares, which is why they are considered for local interventions.

The scheme has two major functions, (1) capturing rain runoff before entering the urban structure and (2) reducing flow to promote retention and prevent flooding. The urban green infrastructure system (

Table 4. Extension and capacity of the proposed urban green infrastructure system.

Strategy	Area (m2)	TRR (m3)	%
Total	4,093,512	1.55	100.00
Vegetated swales	576,000	0.24	15.20
Bioretention basin	80,203	0.04	2.43
French drain	9175	0.01	0.35
Bioretention swale	7619	0.01	0.34
Pervious pavement a/	3,248,955	1.22	78.90
Water park a/	125,518	0.03	1.97
Retention pond	10,000	0.02	1.29
Pervious surfaces/green walls	33,927	0.01	0.62
Infiltration trench	46,042	0.01	0.80

^a/ Both the proposal for pavement and permeable surfaces and the development of the water park form a proposal for specific areas of the city solely to calculate runoff. The volume of water susceptible to retention will vary depending on the surface that is selected for intervention. Source: Based on calculations made for each type of infrastructure and its potential TRR (Total Runoff Retention, see Equation (2)).

) includes an area of 409 hectares that would have the capacity to capture up to 1.55 million m3 of rainwater, which is approximately 2.68% of the annual consumption of drinking water registered for the city in 2018.

In developing countries, it is a common practice that, in public and private buildings, drinking water is used for various purposes, from cleaning to irrigation, as they lack infrastructure designed to recover rainwater and return it to the urban water cycle. This set of strategies is planned to be the basis for the layout and construction of a rainwater drainage network, independent of sanitary drainage, which would allow the collection and distribution of rainwater. Considering that most green infrastructure strategies provide secondary treatment to runoff, the water that circulates through the new rainwater network could be suitable for use in irrigation of green areas or any non-potable urban use, after applying the corresponding analyses for water quality.

4.3. Application of the WSC Model to Aguascalientes City: Non-Structural Mechanisms

Through the implementation of green infrastructure (GI) alternatives described above, the rainwater likely to be harvested would represent a saving of more than 1.5 million m³ of groundwater per year. This amount would increase through the captured volume from peri-urban strategies. In this way, a saving of more than 50% of the total volume of water extracted registered in 2018 would be achieved, making the transition to a WSC even more effective. Although the implementation of intraurban infrastructure does not cause a substantial increase in the potential volume of rainwater catchment, it presents additional potential benefits related to flood prevention, reduction in heat islands, improvement of environmental quality within the urban structure, and the increase in public green areas.

The infrastructure strategies to be implemented in each micro-basin represent a proposal (Figure 3 and Figure 4), but it is not restricted to changes and adjustments. Although all the strategies fulfil the function of capturing, retaining, and/or conducting rainwater runoff, the choice of a different strategy will be determined by the limitations of the terrain or by the construction requirements to which they are subject. Therefore, the compatibility of such strategies will only be limited by the ease or difficulty of the selected area for its implementation or refurbishment.

With the implementation of this proposal, the volume of harvested rainwater could reach 30.206 million m³ of water per year, equivalent to more than 52% of the water produced for Aguascalientes City in 2018. This implies the possibility of reducing dependence on groundwater by almost half, representing a feasible opportunity to achieve the long-term recovery of the Aguascalientes Valley aquifer (GI proposal in Figure 5).

The general proposal for rainwater harvesting will contribute to strengthening the resilience of the city and the peri-urban human settlements of the study area. Although the conducted calculations postulate the implementation of rainwater infrastructure as a highly practical alternative to address water needs for urban use, this is only part of the path towards consolidating Aguascalientes as a WSC. The deployment of structural mechanisms should be added to the development and implementation of non-structural measures promoting social change that helps to ensure the creation of a culture of care and sustainable management of water. This allows, in turn, for certainty about water resources of the region in the long term.

An alternative to contribute to achieving the goals of a WSC is reduction in water consumption patterns, in addition to the implementation of structural measures for rainwater capture, treatment, and reuse. According to the World Health Organization, a supply of 100 L per person, per day, is more than enough for a person to carry out their activities and guarantee satisfactory attention to their needs. Water consumption in Aguascalientes is high, in contrast to what is defined by the WHO, since the daily supply per person is around 200 to 300 L.

According to the WSC model, the purpose of demand reduction is to preserve water and contribute to environmental sustainability by incorporating a variety of water efficiency measures (or reduction in demand) [43]. Taking into consideration the earlier arguments, it is proposed that the daily water supply per person be reduced to the volume proposed by the WHO of 100 L/p/d. This measure would imply a drastic decrease in the water extracted from the aquifer, of up to 60% of the volume generated in 2018 (Reduction in Figure 5). Additionally, it would imply an economic saving, because there is no need to develop infrastructure of any kind. In the first approach, the reduction in the water endowment as such leads to a significant saving in the short term.

Combining both the green infrastructure proposal and the reduction in daily water supply, a scenario could be reached where the consumption of groundwater for the year 2040 would be equivalent to just over eleven million cubic meters, equivalent to a decrease of more than 80% of the total volume registered in 2018 for the city of Aguascalientes (A + B in Figure 5). However, it makes sense that the application of a measure of this type implies a political and social cost that the authorities would not necessarily be willing to assume, which would make a decisive application unlikely. In the last section, we discuss these last ideas by way of conclusion.

5. Discussion and Conclusions

The results show that the application of the WSC model at an urban scale would suggest a considerable reduction in the volume of groundwater extracted for urban consumption in the city, reducing local water stress. With the implementation of this proposal, through structural mechanisms, a rain capture equivalent to more than 52% of the water extracted for consumption by the city of Aguascalientes in 2018 was calculated, suggesting the possibility of reducing dependence on the city’s groundwater in the long term.

The best results regarding the reduction in water consumption could be obtained in a scenario in which green infrastructure is built and policies are applied to reduce the supply of drinking water per household (i.e., non-structural mechanism), but the feasibility of achieving it would be low due to the political cost of implementing drastic measures for the population. It is necessary to conduct education campaigns, environmental awareness, training for personnel of drinking water and sanitation agencies, promotion of governance systems, and citizen participation, all with the aim of improving the relationship between the population, the environment, and the way in which the population makes use of resources. All this would come prior to being able to implement stricter regulatory measures without the population opposing such measures or implying a political cost to decision makers.

This work contributes to the discussion about the potential that the application of the WSC model at an urban scale could have on the pressure levels of aquifers in cities with high water stress in the Global South. Through a theoretical exercise, it was possible to develop a clear and relatively precise approximation of the requirements, benefits, scope, and constraints of the development of infrastructure for rainwater harvesting on an urban scale. The proposal has two central implications. First, in conceptual terms, it opens the possibility of “self-supply” of drinking water to urban locations, especially in semi-arid contexts, which will allow for moving from the idea of a “problem city” to a “solution city”. A criticism of the WSC model is that most of the applied cases correspond to cities belonging to developed countries, which have the economic ability to conduct the construction of infrastructure and the implementation of assistive technology, in addition to the ability to adjust the human, political, and social environment for the correct implementation of the model. Second, the implementation of a WSC model in countries of the Global South presents a series of political and social implications linked to the limits and restrictions that the conventional form of local urban development represents, focused on the development of residential land use and grey infrastructure, which at the same time makes it difficult to incorporate innovative elements such as green infrastructure into planning strategies on the local agenda.

From an urban planning perspective, this paper broadens the discussion on how a functional relationship of a land use compatibility zoning process could change, when including green infrastructure under a WSC scheme, towards one based on a relationship of sustainable use of environmental services. It also underlines the importance of implementing rainwater conservation measures through the integration of green infrastructure as an alternative to traditional urban development. Although it has been pointed out that the implementation of green infrastructure is theoretically more affordable compared with traditional infrastructure, time constraints did not allow estimating the cost of infrastructural development. One of the limitations of the exercise is the available data allowed considering the water demand as a constant, without considering fluctuations in consumption throughout the year. An analysis including information on water consumption by socioeconomic levels, for example, would allow for obtaining a more precise picture of the real water demand and the infrastructure or public policy requirements. Additionally, the analysis of social and political implications is an important field for future research projects, since they make up the non-structural component of the WSC model, essential for its proper implementation and the achievement of the desired goals. Despite the above, the results obtained allow us to glimpse possibilities for the design of urban policies aimed at the configuration of urban space as an alternative response to current challenges of access and water supply.