**1. Introduction**

Water balance approach is used to evaluate availability of drinking water, recharge, water storage and to quantify groundwater and evapotranspiration terms [1–3].Water balance methodology is also used in many water balance studies of lakes to calculate one or more terms of balance, such as precipitation, whose estimate depends on rain gauge placement and spacing; evaporation, estimated by using energy budget, which is the most accurate method; stream discharge and runoff; and the residual of the lake water balance, which is interpreted as the groundwater term [4]. The groundwater contribution can be equal to the water budget residual, or understood as the difference between water input and water output quantity of the lake balance [5]. Groundwater flux into lakes can play an important role in water balances of lakes, especially for shallow lakes without significant tributaries and outflows, in which hydrodynamics are controlled primarily by meteorological conditions and groundwater fluxes [6]. Furthermore, the regime of shallow lakes reacts sensitively to changing conditions, such as variation in water level or in response to heavy storms, which determine changes in lake ecosystems [7]. The turbidity, or transparency, considered a function of lake nutrient status, represents alternative equilibria in shallow lakes as a response to disturbances or changes in external factors (level fluctuation, climate change, water resource management) and to physical and chemical condition (nutrient concentration) [8]. Reference [9] investigated if groundwater could be a corresponding cause of accumulation of phosphorus in the Nørresø lake sediments. They found that groundwater phosphorus input is the same order of magnitude as the total phosphorus deposited in the

**Citation:** Ciampittiello, M.; Dresti, C.; Saidi, H. Water Resource Management through Understanding of the Water Balance Components: A Case Study of a Sub-Alpine Shallow Lake. *Water* **2021**, *13*, 3124. https://doi.org/10.3390/w13213124

Academic Editors: Alban Kuriqi and Luis Garrote

Received: 14 October 2021 Accepted: 3 November 2021 Published: 5 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

shallow lake sediment. The phosphorus concentration in eutrophic lakes is usually thought to derive from agricultural fertilizers and wastewater treatment plants, and the natural release of phosphorus by internal processes is rarely considered and recorded, especially if it is thought to be related to groundwater transport [9]. All of these reasons, the knowledge of hydrological balance and each of its terms for shallows lakes, if they are eutrophic and if they are mostly fed by groundwater, are the basis for every action of water management. In fact, the assessment of the impacts of long-term climate variability on water balance terms by using time series of meteorological variables is crucial for the management of water resources, especially for shallow lake systems [10]. Impacts of water resource management can be particularly marked, but also climate, either on a local or catchment scale, is of great importance for lake hydrology as it determines both the inputs and outputs of water [11]. In this framework, a comprehensive understanding of the interaction between surface water and groundwater is largely needed to develop effective policies of water resource management and protection, especially if we consider that the water level fluctuation may have an overriding effect on the ecological functioning of ecosystems [11]. If small lakes are principally fed by groundwater, it is necessary to understand the relationship between rainfall, level fluctuations, and the aquifer. The hydrogeological catchment is not often well known, and to understand climate change impacts on small lakes fed by groundwater, it is important to investigate the origin, direction, water quality and quantity, susceptibility, and timing of groundwater recharge [12]. Water resource management has to take into account other variables, including climate change and variation in water demand—industrial and agricultural—and in water supply that can affect water balance and ecosystems [13].

To analyze the functioning of hydrogeological systems in a shallow lake where groundwater is the main source of water and to analyze the impact of climate change on the lake, consequently proposing correct management of the water resource, we considered Lake Candia, a morainic shallow lake. For analyzing the hydrogeological system, water balance was calculated using soil water balance and determining the groundwater term as the difference between water input and output. Additionally, the trends for each term of the water balance and the climate change of main meteorological parameters were evaluated. Finally, by using the most significant terms of the water balance, a regression analysis was developed to define correct water resource management.

#### **2. Materials and Methods**

The Ivrea Morainic Amphitheatre (IMA) was defined as the most remarkable amphitheater of the Alpine context, due to its clearly expressed morphological arrangement [14]. Its most typical elements are (i) an exceptionally regular and very long (16 km) lateral moraine, named the Serra d'Ivrea; (ii) a very large fluvial plain occupying the internal depression; and (iii) a wide sector of rocky reliefs (21 km2) connected to sub-glacial morphologies, named the Colli d'Ivrea, cropping out above the internal plain [15]. After the glacier withdrawal, the presence of morphological barriers and low-permeability hydrogeological interfaces created optimal conditions for the surface accumulation of meltwater within the IMA internal depression, with consequent formation of several shallow lakes. Just north of Ivrea there are the "Six Lakes", the largest of which is Lake Sirio, the right lateral moraine hosts lakes Alice and Meugliano, Candia Lake and the smaller Maglione and Moncrivello lakes lay between the hills that form the front moraine.

A close interaction between this territory and human activities has developed over time. A good knowledge of resources (water, geological, hydrogeological) and their vulnerable assets is fundamental for safeguarding and valuing this alpine area [16]; the hydrogeological catchment of the Ivrea amphitheater represents an important water resource for the territory, both for the environment and for human activity.

Lake Candia (Figure 1) is the second largest lake of the IMA and it is likely fed primarily by groundwater and rainwater, rather than by the small canals running along the surrounding hillslopes. A small outlet links the lake to the Dora Baltea River. Water exchange is slow and the concentration of nutrients is consequently high, due to the runoff from the surrounding agricultural fields and to the natural lake conditions. Since 1995, the lake and the wetlands have been protected as a natural reserve, the first provincial park in Italy. Furthermore, the park was declared a site of community importance according to the European Union "Habitat" directive. Lake Candia will also soon be included in the list of protected wetlands, according to the Ramsar Convention (http://www.park s.it/parco.lago.candia/Eindex.php, accessed date: 2 November 2021). The definition of adequate water management strategies for these particular ecosystems, taking into account the impact of climate change on these lakes, can offer tools for the protection of ecosystems and recommendations for sustainable development. The Lake Candia watershed covers 8.91 km2 and has a mean altitude of 266 m a.s.l. Maximum depth of the lake is 7.7 m, average depth is 4.7 m, and volume is 0.007 km3.

**Figure 1.** Catchment of Lake Candia.

The Lake Candia catchment is characterized by intense agricultural land use, where the arable portion is the largest. The surplus water of the agricultural network is discharged directly into the lake. The lake is fed primarily by groundwater and by rainwater falling directly on its surface; runoff from the watershed is the third source in order of importance, with characteristics varying according to the amount of rainfall and the season [17]. The lake's outflow, the Fosso Traversaro, with which the watershed comes to an end, is in the southwestern part of the lake, off-center from the more northerly orientation of the lake. The discharge is regulated by a weir (Figure 1).

Following Köppen's classification [18], the lake area has a temperate sub-continental climate, with daily average air temperatures ranging from −2 ◦C in the coldest month (January) to 30 ◦C in the hottest (July). The rainfall regime is western sub-littoral, according to the climate classification reported by [18], and is characterized by two maxima and two minima, with the highest maximum in spring and the lowest minimum in winter, with mean annual values around 900 mm.

A large variety of geophysical surveys was conducted on Lake Candia during the last decade [12,19].

Some equipment was installed in 1987 on the southwestern shore of Lake Candia (Figure 1) to measure the main meteorological parameters, such as rainfall, air temperature, wind direction and speed, solar radiation (direct and reflected), humidity, pressure, and lake level. In April 1987, a trapezoidal Cipoletti weir was built at the outlet (Figure 1) to regulate the discharge so that water would not flow out of the lake if the water level fell below 30 cm. The data from the weather station available for climate analyses are recorded continuously; the station is operated by the Regional Protection Agency (ARPA)

of Piedmont Region (http://www.arpa.piemonte.it/rischinaturali/accesso-ai-dati/anna li\_meteoidrologici/annali-meteo-idro/banca-dati-meteorologica.html, accessed date: 2 November 2021). The weir discharge data are in direct relation with the level of the lake, so that a continuous reading of the levels gives a continuous discharge datum for the outflow.

The analyses were carried out using meteorological and discharge data recorded at the automatic measuring station and were used for: (i) evaluating water balance to determine the amount of groundwater; (ii) evaluating the trend of each component of water balance (rainfall direct on lake; entrance, the component that comprises runoff, exceeded irrigation, and irrigation runoff; groundwater; discharge from the emissary; and volume variation); (iii) calculating the presence of break point or changes in the behavior of each component; and (iv) investigating the regression of water balance terms to understand their relationship, including possible effects among each other and for improvement of water research management.

#### *2.1. Water Balance*

Using monthly data from 1993 to 2019, a two-step approach was used to calculate the volume of groundwater and thus evaluate its importance in the Lake Candia hydrological regime. As the study area contains a water body (Lake Candia), which exercises its hydraulic action on the magnitudes of the water balance terms, the continuity equation was applied first to the lake and then to the whole basin.

The continuity equation applied to the lake follows:

$$\rm{P\_{L\overline{C}} + R\_{\overline{s}} + IR\_{\overline{E}} + R\_{\overline{R}} + Q\_{\overline{S}} = E\_{L\overline{C}} + \Delta H + Q \tag{1}$$

where:

PLC is direct rainfall on the lake surface and on the part of the reed bed connected to it;

RS is the surface runoff;

IRE is the portion of irrigation water that is not used and enters the lake directly;

RIR is the runoff from irrigation;


If we look at the whole equation [20], the water entering the lake is made up of total rainfall P, transformed into rainfall on the lake (PLC) and net rainfall (RS); irrigation water, which must be taken into account due to the presence of a number of cultivated fields, and a further addition from groundwater (QS), which is thought to feed the lake due to the existence of resurgences within the lake and the hyporheic flow [21]. With respect to the general equation, the outgoing water comprises the evaporation of both water body and reed bed (ELC); variations in the level of the lake (ΔH), taken as increases and reductions of its volume; and the discharge measured at the weir (Q) located on the outlet. At this initial stage, neither evapotranspiration from vegetation in the watershed nor variations in soil moisture content have been taken into account.

The criteria adopted to obtain each of the terms of the balance are given below.

PLC—Rain falling directly on the lake and the reed bed.

This is the portion of precipitation falling directly on the lake and the reed bed, most of which grows with its roots in the lake or floating [22], thus without being intercepted by plants or soil. It was calculated by multiplying the rainfall depth by the lake area and the reed bed area.
