**1. Introduction**

Coastal aquifer systems nowadays present an ever-declining quantity and quality degradation as they are intensively used for irrigation purposes. This is especially the case for the aquifer systems located in arid and semi-areas like the Mediterranean basin, where fertile soils and favorable climatic conditions host the productivity of the agricultural and food sectors. It has been predicted that climate change will have a greater impact on the water resources of the Mediterranean areas, and it will alter the water cycle's temporal and geographical distribution. Water scarcity will be advanced in intensity and magnitude while crop yields are expected to decline [1]. Whenever seawater intrusion is an area of concern for water resources in irrigated agriculture as well, it is quite crucial to examine the potential effects of climatic change in coastal arable watersheds [2]. In coastal watersheds where agriculture is the main economic activity, the managemen<sup>t</sup> of water resource hazards stem from the absence of water storage works, the large amounts of groundwater abstractions that also provoke seawater intrusion, and the large amounts of fertilizers to maximize yields. Irrigation with salinized water causes physiological drought to crops, with effects similar to climatic drought events with regard to crop productivity [3,4]. In general, estimating water resource productivity at the watershed/basin scale is not an extensively studied task either under climate change and/or salinity effects on productivity. The aim of this work is to examine the climate change impacts on water resources efficiency and crop productivity under agronomic and irrigation scenarios, as well as the adaptation potential under water resource development projects at the Mediterranean Almyros basin,

155

**Citation:** Lyra, A.; Loukas, A. Water and Nitrogen Use and Agricultural Production Efficiency under Climate Change in a Mediterranean Coastal Watershed. *Environ. Sci. Proc.* **2023**, *25*, 23. http://doi.org/10.3390/ ECWS-7-14180

Academic Editor: Rodrigo Maia

Published: 14 March 2023

**Copyright:** © 2023 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/).

a coastal region in Thessaly, Greece, considering, also, the salinity effects on crop yields. The water resources have been simulated based on two climatic Representative Concentration Pathways (RCPs), namely, RCP4.5 and RCP8.5, using an Integrated Modeling System (IMS) formed by Lyra and colleagues for implementation on coastal agricultural watersheds [2,5]. The indices of Standardized Chloride Hazard (SCHI), Crop Water (CWP) and Economic Water Productivity (EWP), and Nitrogen Use Efficiency (NUE) have been employed to analyze water resource adaptation and agronomic alternatives. The findings indicate the water resources' potential for adaptation as well as their agronomic productivity under climate change.

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

### *2.1. Study Area*

Almyros basin is located in central Greece, and it is an agricultural coastal basin where wheat, alfalfa, cereals are the main cultivars, while cotton, olives trees, maize, vegetables, orchards, and vineyards are also cultivated. Given the absence of existing freshwater reserves, groundwater has been solely used for irrigation. The hydrology of the region is described by streams with intermittent flows and semiarid climatic conditions [2]. The groundwater in the study basin has been seriously affected and polluted by contaminants from nitrogen leachates and chloride ions from saltwater intrusion. Recently, an urban water supply reservoir, the Mavromati reservoir, has been built, and an irrigation water supply reservoir, the Xirias reservoir, is under construction (Figure 1). Furthermore, a greater irrigation water reservoir has been studied, the Klinovos reservoir (Figure 1).

**Figure 1.** Elevation of the Almyros basin, principal streams, Almyros aquifer system, reservoir locations, and irrigated regions.

#### *2.2. Climate Change and Integrated Modelling System*

The simulation is performed for climatic model ensembles based on Med–CORDEX models. Precipitation and temperature ensemble timeseries for the RCPs 4.5 and 8.5 have been bias-corrected using Quantile Mapping. Bias–correction calibration took place during the period 1971–2000, and validation during 2001–2018, counting on observed precipitation and temperature for the studied area, as in a recent earlier study [2]. The simulation of water resources of the study basin was performed using the calibrated and high efficiency Integrated Modelling System consisting of interlinked/coupled models for surface hydrology (UTHBAL), reservoir operation (UTHRL), agronomic schedules/crop growth/nitrate leaching processes (REPIC), groundwater flow (MODFLOW), nitrate transport (MT3DMS), and salt wedge/seawater intrusion (SEAWAT) composed by Lyra and associates [5]. Groundwater simulations for the water table and the nitrate and chloride concentrations started from 1991 because of the availability of historical hydrogeological data.

#### *2.3. Water Resources and Agronomic/Crop Scenarios and Strategies*

Strategy A, in which only groundwater is used for irrigation/urban water supply (baseline historical strategy), and Strategy B, in which surface water reservoirs have been developed and used along with groundwater abstractions for irrigation/urban water supply, have been developed. Several agronomic and irrigation scenarios have been developed and simulated with the Strategies A and B, namely historical irrigation and nutrient practices (A0/B0), deficit irrigation and historical nutrient practices (A1/B1), rainfed agriculture and historical nutrient practices (A2/B2), deficit irrigation and reduced nutrient practices (A3/B3), and deficit irrigation and rainfed agriculture and reduced fertilization (A4/B4).

#### *2.4. Salinity, Chlorides Concentration, and Crop Yield*

The crop yield is steady within a given range of soil salinity, but, after reaching a maximum tolerance level, the crop output decreases in an idealized, simple linear trend. The electrical conductivity of the water (ECw) can be approached with a concentration coefficient (X) that depends on the leaching physiology of the cultivated crops, and underlying soil, and the conductivity of soil extract (ECe) [4]. In order to take into account, the saline implications of the seawater intrusion on agricultural output, the crop yields are adjusted using a relative percentage of yield performance [3]. Electrical conductivity and chloride concentration observations performed by various public and private organizations and former studies, as described in [5], span from 1991 to 2015. Chlorides range from 4 to 1432 mg/L, and ECw ranges from 0 to 5 dS/m.

#### *2.5. Agronomic Indices and Standardized Chloride Hazard Index (SCHI)*

The Standardized Chloride Hazard Index (SCHI) has been used for detecting and characterizing the adaptation of using coastal groundwater for irrigation and the possible impacts of its use on the crop yield, as follows:

$$\text{SCHI} = \left( \text{Cl}\_i - \overline{\text{Cl}} \right) / \sigma\_{\text{Cl}} \tag{1}$$

where *Cli* is the chloride concentrations as simulated by SEAWAT in a monthly timestep, *Cl* is the monthly average, and *σCl* is the standard deviation of the chloride concentrations. The agronomic indices are based on the simulated crop yields by the REPIC model regarding the various alternatives. The index (CWP) quantifies the yield produced for every cubic meter of water applied. The Economical Water Productivity (EWP) indicator determines the performance at each cubic meter of water supplied, and, based on published commodity producer prices for agriculture by OECD–FAO until the 2030s [6], the index scores for the future periods have been projected and estimated. The Nitrogen Use Efficiency index (NUE) quantifies the yield produced for every kg of nitrogen applied.
