Impacts of irrigation and nitrate fertilization scenarios on groundwater resources quantity and quality of the Almyros Basin, Greece

Irrigation and nitrate fertilization scenarios were combined and simulated for crop water irrigation demands and nitrogen applications based on experiments on crop cultivation practices. Two irrigation practices (deficit irrigation and rainfed conditions) were applied to the main crop types of the Almyros Basin, a coastal basin located in Thessaly, Greece. The Almyros groundwater system suffers from progressive water balance deficit, nitrate contamination and seawater intrusion due to groundwater abstractions for agricultural irrigation to cover crop water demands in the dry season. The impacts of the irrigation and nitrate fertilization scenarios on groundwater resources quantity and quality were simulated using an Integrated Modelling System consisting of models of surface hydrology (UTHBAL), groundwater hydrology (MODFLOW), crop growth/nitrate leaching (REPIC), contaminant transport (MT3DMS), and seawater intrusion (SEAWAT), for the historical period of 1991–2018. The results of the scenarios were evaluated with the indicators of Crop Water Productivity (CWP) for crop yields and irrigation water, Partial Factor Productivity (PFP) for Nitrogen Use Efficiency, and Economic Water Productivity (EWP) for the gross profits of the irrigation water.


GRAPHICAL ABSTRACT INTRODUCTION
The impacts of irrigation and fertilization practices on groundwater resources in semi-arid regions is one of the most important water supply problems of irrigated agriculture and water quality deterioration of aquifer systems (Fernández García et al. ; Lyra et al. ). In coastal water systems, over-pumping for irrigation water supply, the absence of significant surface water reservoir infrastructure (and for this reason the use of surface water), and excessive nitrogen applications for crop growth improvement, have a number of implications for groundwater, notably the water table descent, nitrate contamination and seawater intrusion (Lyra et al. ).
The continuing exacerbation of water deficit and groundwater quality poses a threat on the sustainability of irrigated agriculture in coastal regions. The water quality limit for drinking water regarding nitrates is 50 mg/L (NCESD ) while for chlorides it is 250 mg/L. According to the European Directives, chloride concentrations in groundwater systems can be between 24 mg/L and 12,300 mg/L depending on aquifer characteristics (EU ).
An integrated approach to the sustainability of crop yields and to the control of nitrate fertilization, groundwater abstractions and seawater intrusion is essential for protecting the viability of natural resources. The key points of such an approach are the understanding and assessment of water budget fluxes and safe water quality supply for irrigation and drinking water consumption (Loukas ).
In regions where the largest amount of water resources is consumed for agricultural production and the water balance is negative, the estimation of productivity efficiency is of paramount importance. The trend of marketable crop production is directly linked with irrigation water application, while the latter affects the availability of nitrogen in soils and nitrogen leaching to groundwater (Ullah et al. The scope of this study is to evaluate the impacts of irrigation and fertilization scenarios on groundwater resources, as well as their benefits compared to the current agricultural and water management practices in the Almyros Basin, a coastal agricultural basin in Thessaly, Greece. The evaluation has been performed with the application of an Integrated Water Resources Modelling System developed by the authors (Lyra et al. ) and applied to the study area for the simulation of the quantity and quality status of groundwater and crop yields for various scenarios of irrigation and nitrogen fertilization. The results of each alternative scenario were quantified using the indicators of CWP, NUE and EWP for the simulation period of 1991 to 2018.

Study area and database
The main land use of the Almyros Basin is for agricultural fields, with a small percentage of the basin being used for urban, sub-urban and non-cultivated areas. The hydrography of the basin is characterized by ephemeral streams and absence of significant surface water storage works. The cul-  Figure 1. The cultivated agricultural lands are totally irrigated, due to the absence of surface water storage works, with water obtained from groundwater. Groundwater also supplies water for urban use and it is taken into account in this study. However, the urban water supply volume is not as high as the water volume abstracted for irrigation. The annual agricultural water supply accounts for 29.7 hm 3 , on average, while the mean annual urban water supply volume reaches 1.8 hm 3 (Lyra et al. ).

Agricultural water supply scenarios
The agricultural water demand is defined by the crop water demands for crops cultivated in the Almyros Basin. More than two thirds of the area is permanent cropland, irrigated by groundwater abstractions with private-owned wells. The number of irrigation water wells has been estimated at 2,044, indicating intensive agricultural activities (Lyra et al. ). The crop water requirements were estimated in   Table 1.

Drinking water supply
The

Indicators
The CWP and EWP indices regarding the irrigation water applied and the grain yield were implemented for the evaluation of the simulation results for the Almyros Basin.
The CWP for irrigation water is calculated by Equation (1

MODFLOW model
The groundwater fluxes were simulated for the period of 1991 to 2018 for the scenarios of deficit irrigation (S1, S3) and deficit and rainfed conditions (S2, S4). The simulated groundwater heads for the four scenarios (i.e. S1, S2, S3, S4) were compared with the respective simulated results for the baseline scenario (S0) for the end of the simulation period (2018) in

MT3DMS model
The nitrate fluxes were simulated by the MT3DMS model for the period of 1991 to 2018. The simulation was performed for all four scenarios (S1, S2, S3, S4). The differences between the simulation results for the baseline scenario, S0, and scenarios S3 and S4 are shown in

SEAWAT model
The chloride fluxes were simulated using the SEAWAT model for the period of 1991 to 2018 and the simulation was performed for all four scenarios (S1, S1, S3, S4).
The differences of the simulation results of scenario S3 and scenario S4 from the baseline scenario S0 are depicted in Figure 5

Water budget
The average monthly (Figure 6(a)) and annual (Figure 6(b)) water budget of the Almyros Basin aquifer for the deficit irrigation scenarios (S1, S3), and the deficit irrigation and rainfed cultivation scenarios (S2, S4), were calculated and compared to the current historical groundwater regime of the Almyros aquifer for the simulation years 1991-2018.
Due to the large extent of the spatial distribution of cereals and wheat, the deficit irrigation scenarios (S1, S3) show a monthly higher positive inflow, which is attributed to the irrigation return flows, as compared to the scenarios of deficit irrigation and rainfed cultivation (S2, S4), when groundwater is only recharged by the infiltration of precipitation. The annual water deficit of the baseline scenario S0 is 12.02 hm 3 (Lyra et al. ), minimized in the deficit irrigation case to 2.3 hm 3 (scenarios S1 and S3), and in the deficit irrigation and rainfed cultivation case (scenarios S2 and S3) to 2.8 hm 3 . The annual water deficit for the deficit irrigation and rainfed cultivation case is larger than the water deficit for the deficit irrigation case alone, due to the limited natural recharge and absence of irrigation return flow for a large extent of the aquifer area. Nevertheless, both practices have a significant positive impact on the water balance.

Crop water productivity
The CWP indicator was estimated using Equation (1) Figure 7(a). The CWP I is increased for the deficit irrigation conditions (i.e. S1 and S3) for all crops in the Almyros Basin.

Partial factor productivity
The PFP indicator was estimated using Equation (3) for the evaluation of the efficiency of nitrogen use on crops on the simulated scenarios for baseline fertilization (S1, S2) and reduced fertilization (S3, S4). In the reduced fertilization practices in all crop types, except for alfalfa, PFP is increased, especially for cereals, maize, vineyards, and wheat. The comparative percentages of the variations of the indicator from the baseline scenario S0 scores are presented in Figure 7(b).

Economic water productivity
The EWP indicator was estimated using Equation (2)

CONCLUSIONS
Irrigation water and nitrogen fertilization applications were simulated for the assessment of their impact on the groundwater quantity and quality of the Almyros Basin. The Integrated Water Resources Modelling System applied has been expanded to include the simulation of agronomic and irrigation practice alterations. All the scenarios (S1, S2, S3, S4) have a positive impact on the water balance, and the water deficit is reduced significantly by 9.7 hm 3 in scenarios S1 and S3, and 9.2 hm 3 in scenarios S2 and S4.
The larger water deficit in scenarios S2 and S4 is caused by the absence of irrigation return flows in the rainfed Figure 6 | Average monthly (a) and annual (b) water budget of the Almyros aquifer for the baseline scenario S0 and the scenarios of deficit irrigation (S1, S3) and deficit-rainfed irrigation (S2, S4).

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A. Lyra et al. | Impacts of irrigation and nitrate fertilization scenarios on groundwater resources Water Supply | in press | 2021 Corrected Proof areas. Therefore, the latency of seawater intrusion is succeeded in the northern coastline area under all scenarios, and in scenarios S2 and S4 the aquifer's chloride concentrations are more reduced than in scenarios S1 and S3.
The scenario practices show positive impacts on nitrate contamination, especially in scenarios S2 and S4. The nitrate concentrations are reduced in large extent and in all cases, but the irrigated and fertilized maize adjacent to areas of rainfed crops cause local assimilation of nitrates and have a negative impact on the aquifer's water quality.
The crop yield results show that vineyards are susceptible to deficit irrigation, while cereals, maize and wheat are resilient to changes in agronomic practices. All crops had an improved CWP I in all scenarios, as compared to baseline scenario S0, reaching maximum scores in baseline fertilization practices. Consequently, the EWP I of the crop pattern reached an optimum score of 1.29 €/m 3 in scenario S1. However, the crops' PFP was increased when reducing nitrate fertilization in scenarios S3 and S4, especially for cereals, maize, vineyards, and wheat. In total, the results indicate that the higher positive impacts on groundwater quantity and quality are achieved in scenarios S2 and S4. The further improvement of groundwater quantity and quality may be achieved by changing the

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.