**3. Results-Discussion**

### *3.1. Salinity Impacts on Crop Yield*

Based on relevant measurements, it was possible to configure useful information for the relationship of chloride concentrations and electrical conductivity in the Almyros aquifer system. The linear regression equations that connect the two variables are:

$$\rm{EC\_w(dS/m)} = 0.0032 \times \rm{Cl\_w (mg/L)} + 0.6212 \quad (R^2 = 0.9) \tag{2}$$

$$\text{Cl}\_w \, (\text{mg/L}) = 272.32 \times \, \text{EC}\_w (\text{dS/m}) - 159.17 \quad (R^2 = 0.9) \tag{3}$$

Additionally, the weighted mean leaching fraction and concentration coefficient of the Almyros aquifer are 0.1 and 2.1, respectively. Table 1 presents the rating of seawater intrusion and salinity hazards primarily for the Almyros groundwater body based on the classifications by [3]. According to the simulated chloride concentrations by the SEAWAT model and the crop yields by the REPIC model, Table 2 shows the mean Relative Yield changes caused by salinity for the two RCPs (4.5 and 8.5). Crop production would be reduced under historical irrigation practices regardless of the development of surface water reserves by –0.3% in RCPs 4.5 and 8.5 in 2019–2050 due irrigation groundwater salinity. The implementation of deficit and rainfed agriculture has a stronger positive influence on the decrease of salinization, and, thus, the contribution of the alteration of agronomic practices is evident with a 0.3% gain in productivity in 2019–2050. However, rainfed agriculture is more efficient to maintain and increase the crop production in 2051–2100 since it is not affected by the operation of reservoirs and the cessation of groundwater abstractions and groundwater salinity.

**Table 1.** Classification of the electrical conductivity of saturated extract (ECe), the pumped groundwater for irrigation (EC w), the chloride concentrations (Cl w) of water, and the Standardized Chloride Hazard Index (SCHI) with regard to the salinity hazards.


**Table 2.** Relative Yield changes of the future periods from the historical period for the water resources and agronomic scenarios for both RCPs (4.5 and 8.5).


#### *3.2. Agronomic Efficiency Indices and Water Resource Adaptation for Seawater Intrusion*

The agronomic efficiency indices and the water resources adaptation index for seawater intrusion have been estimated and summarized for the time periods of 1991–2018, 2019–2050, and 2051–2100. The SCHI index has been calculated based on the results of the Integrated Modelling System (IMS) and especially the SEAWAT model. SCHI index scores range for all scenarios and time periods from low salinity hazard, −1.14 in the historical period, to almost extremely high salinity hazard, 1.42 in the future period, proving the downgrading of the groundwater from non-saline to very saline. In 2019–2050 under historical and deficit irrigation practices, groundwater use for irrigation will pose low-moderate salinity hazards, whereas rainfed agriculture/deficit irrigation will pose moderate rates in both Strategies A and B and in both RCPs. However, in 2051–2100, the situation will be reversed. Low-moderate salinity hazards will appear during rainfed agriculture/deficit irrigation and moderate salinity hazards for the remaining scenarios for both RCPs.

The agronomic indices have been calculated based on simulations by the Integrated Modelling System (IMS). Crop yields are calculated by the REPIC model and the groundwater abstractions and quality by the MODFLOW and SEAWAT models. Crop yields are multiplied to their relative change, due to salinity impacts of the groundwater as irrigation water, to obtain the simulated production considering the salinity effects. Commodity prices, for use in the EWP index of the crop pattern, were estimated on a weighted spatial average of 0.33 €/kg crop yield in 1991–2018, 0.49 €/kg in 2019–2050, and 0.65 €/kg in

2051–2100. Figure 2 depicts the Crop Water Productivity (CWP), Nitrogen Use Efficiency (NUE), and Economical Water Productivity (EWP) indices' values. CWP and EWP index scores thrive in Strategy B than in A, proving the benefits of surface water reserves on water efficiency. CWP weighted averaged values range from 6.4 tn/m<sup>3</sup> to 19.4 tn/m<sup>3</sup> in RCP8.5, and from 6.5 tn/m<sup>3</sup> to 17.1 tn/m<sup>3</sup> in RCP4.5. CWP variations are approximately 3 to 4 tn/m<sup>3</sup> between deficit irrigation and rainfed agriculture/deficit irrigation practices.

**Figure 2.** Standardized Chloride Hazard Index (SCHI), Crop Water Productivity (tn/m3), Economic Water Productivity (€/m3), and Nitrogen Use Efficiency for the Strategies A (groundwater resources—right) and B (for groundwater resources and reservoirs storage water—left); for the agronomic/irrigation scenarios 0: historical practices of irrigation and fertilization; 1: deficit irrigation and historical fertilization; 2: deficit/rainfed irrigation and historical fertilization; 3: deficit irrigation and reduced fertilization; and 4: deficit/rainfed irrigation and reduced fertilization. Climate change is shown for the average values of the periods 1991–2018, 2019–2050, and 2051–2100 in (**<sup>a</sup>**,**c**–**<sup>e</sup>**) for the RCP4.5; (**b**,**f**–**h**) for the RCP8.5.

The groundwater abstractions and salinity effects cause the productivity of water to decline mostly during 2019–2050. EWP scores follow the distribution of CWP. EWP ranges from 1.2 €/m<sup>3</sup> to 10.5 €/m<sup>3</sup> in RCP8.5 and to 12.2 €/m<sup>3</sup> in RCP4.5 for Strategy A. For Strategy B, EWP ranges from 5.4 €/m<sup>3</sup> to 22.9 €/m<sup>3</sup> in RCP8.5 and from 6.1 €/m<sup>3</sup> to 21.8 €/m<sup>3</sup> in RCP4.5. EWP will be increased after 2050, but this would occur possibly due to the increase of future commodity prices. NUE index scores show a declining trend in ongoing fertilizer practices until 2100 in both Strategies and RCPs. NUE values in deficit irrigation and rainfed agriculture/deficit irrigation form a V–shaped evolution with

the lowest points to show in 2019–2050 in both Strategies and RCPs. NUE maximizes under the alternative practice of reduced fertilization in both strategies and RCPs. In reduced fertilization, NUE gets scores higher than 100 kg of crop yield per kg of nitrogen applied, while in other alternatives the values range from 71.2 kg yield/kg nitrogen to 93.2 kg yield/kg nitrogen.
