Quantification of water requirement of some major crops under semi-arid climate in Turkey

Study area

The study was carried out in the Gaziantep provinces of the Southeastern Anatolia Region. The Gaziantep province is located between 36°28′ and 38°0′ east longitudes and 36°38′ and 37°32′ north latitudes (Fig. 1).

Gaziantep province has the characteristics of transition climate between Mediterranean and the continental climate. The winter season is cold and rainy, while the summer months are quite hot and dry. The highest and lowest temperatures of the year vary between 44 °C and −17.5 °C, and the annual average temperature is 15.1 °C. Long-term annual average rainfall is 463 mm of which most rainfall occurs in winter. The rainfall is quite low in the summer season when evapotranspiration is high and the plant water requirement is at the most (Aydın, 2004).

Total land area of Gaziantep province is 682,280 ha, and 50.6% (345 415 ha) of the lands consists of agricultural lands. Orchards have the largest share (62%) in agricultural lands, followed by field crops (35%) and vegetables fields (3%). Pistachio has the largest share (65%) in orchards, followed by olive (21%), vineyard (8%) and other fruit (6%) growing lands (Gaziantep Directorate of Provincial Agriculture and Forestry, 2021). Therefore, pistachio, olive, grape and almond cultivation has an important place in the regional economy (Table 1). The province ranks the first in the coverage area of pistachio orchards and the amount of pistachio production in Turkey (Aslan, 2017).

Description of CROPWAT 8.0 model

The CROPWAT 8.0 model is a windows-based software used to determine ET_c, developed by the contributions of the Food and Agricultural Organization (FAO), Land and Water Development Division of Italy, Irrigation and Development Studies of Southampton and Egypt National Water Research Centre (Balasaheb & Sudarsan Biswal, 2020). The model allows to carry out simulations under water stress conditions and to estimate possible yield decrease in response to a decrease in irrigation water (Verma et al., 2019). The model is effectively used as a decision support system in irrigation project design studies and in the development of irrigation programs due to ease of use. Likewise, this model is used in different ecological conditions to estimate the evapotranspiration rate, to determine irrigation requirements and deficit irrigation for different crops (Moseki, Murray-Hudson & Kashe, 2019). In addition to the calculation of crop water requirement, the model can be used to prepare irrigation programs under different irrigation conditions and different plant growth conditions, to develop more efficient irrigation methods, and to evaluate crop yield under rainfed conditions or adequate irrigation conditions (Kartal et al., 2019).

The agronomists, irrigation and water resources engineers engaged in agricultural production can also use the same model to calculate ET_o (Mehta & Pandey, 2015). The aforementioned model considers the grass as the reference plant, which does not have water restrictions, fully covers the ground surface and has good growth. ET_c values of the crop are calculated by using Eq. (2) where the ET_o values are multiplied by the factor K_c for the local crops under study. ET_o values are derived by the CROPWAT 8.0 model based on the Penman-Monteith method given in Eq. (1).

Data requirements

The CROPWAT 8.0 model requires four basic data groups including climate parameters (maximum, minimum and average temperatures, relative humidity, sunshine, wind speed), rainfall, soil (heavy, moderate, light), and the characteristics of the crop studied. The climate data required for the model can be obtained using the CLIMWAT 2.0 software, which includes the climate data measured from different parts of the world, as well as the data of local meteorology stations (Ewaid, Abed & Al-Ansari, 2019).

Other parameters, required for the CROPWAT 8.0 model, like crop coefficient (K_c), yield response factor (K_y), crop growth period values provided in Tables 2–3 for pistachio were compiled from local studies (Aydın, 2004). In addition to aforementioned properties, others such as rooting depth and critical depletion fraction (P) for olive, almond and grape plants were manually compiled from FAO56 and other related literature (Food and Agriculture Organization, 2002 and Ministry of Food, Agriculture and Livestock of Turkey, 2017). The climate and soil parameters of Gaziantep province including rainfall and soil properties and the data on the plant provided in Tables 2 and 3, were used in the CROPWAT and CLIMWAT softwares to calculate the ET_o and CWR values,

The agricultural soils of Gaziantep Province have a medium-heavy texture (Kalkancı& Şimşek, 2020); thus, soil texture for the agricultural soils in this study were considered as medium textured in the solution of the CROPWAT software.

Crop evapotranspiration (ET_c)

The actual ET_c values of the crops studied are calculated by the use of two parameters namely the ET_o and the factor crop coefficient (K_c) as given in Eq. (2). ET_o values were calculated bythe CROPWAT 8.0 model using the Penman-Monteith equation (Allen et al., 1998). Most parameters in the FAO56-Penman-Monteith model can be measured directly, while some parameters can be calculated using related equations of the subjects. (1)${\displaystyle {\mathrm{ET}}_{\mathrm{o}-\mathrm{PM}}=\frac{0.408\Delta \ast \left({\mathrm{R}}_n-\mathrm{G}\right)+\gamma \left(\frac{900}{T_{\mathrm{mean}}+273}\right)u_2\left(e_s-e_a\right)}{\Delta +\gamma \left(1+0.34u_2\right)}}$ where; ET_o is the reference crop evapotranspiration (mm d⁻¹); R_n is the net radiation (MJ m⁻²), G is the soil heat flux (MJm⁻²), T_mean is the average air temperature (°C), U₂ is the wind speed at 2 m height (m s⁻¹), e_s is the saturation vapor pressure (kPa), Δ is the slope of vapor pressure curve (kPa °C⁻¹), γ is the psychrometric constant (kPa °C⁻¹) (2)${\displaystyle {\mathrm{ET}}_{\mathrm{c}}={\mathrm{K}}_{\mathrm{c}}\times {\mathrm{ET}}_{\mathrm{o}}.}$

The coefficient K_c, used to determine ET_c, is the result of combined effects of transpiration from the plant surface and evaporation from the soil surface and reflects the common characteristics of both surfaces (Shah, Suryanarayana & Parekh, 2017).

Crop coefficient (K_c)

The crop coefficient (K_c) is defined as the ratio of the ET_c to the ET_o (Food and Agriculture Organization, 2002). The FAO-56 model is based on the concepts of ET_o and crop coefficient (Poblete-Echeverría & Ortega-Farıas, 2013). Four important and different periods of crops are taken into account in the calculations of K_c based on ET_c. (i) The initial stage is the period from seed sowing to germination and 10% ground coverage; (ii) the growth period is between the end of the initial stage and the period when the crop completes the growth and the percentage of ground cover reaches 70–80%; (iii) mid-term is the stage when crop growth is completed or the crop fully grows and matures with the onset of flowering; (iv) the period between the maturation stage and the harvest is considered the end period (Çetin, 2013).

The K_c coefficients were used to calculate ET_c values via ET_o values based on the FAO56-PM equation (Eq. 1). The K_c values of local crops pistachio (Aydın, 2004), olive, grape and almond (Ministry of Food, Agriculture and Livestock of Turkey, 2017), were compiled from the literature considering the plant growth stages (initial, mid, end stages).

Crop water requirement (CWR)

Semi-arid climate zone characteristics of the region in mind, pistachio, olive, grape and almond are the main agricultural and economical crops of the Southeastern Anatolia Region of Turkey and require an efficient irrigation program. The CWR, including the losses and the ET_c, the amount of net irrigation water requirement (NIRs) and irrigation schedules were determined for the same crops by using CROPWAT 8.0 software. The inputs of the model are long-term annual average rainfall and effective rainfall values both obtained by the USSC method, and other soil and plant parameters like K_c, K_y etc. The monthly ET_o values calculated with the CROPWAT model using Gaziantep climate data are given in Table 4.

Irrigation scheduling and net irrigation requirement

The expected benefit from irrigation can be obtained only if the amount of water needed by the crops and proper irrigation program are determined in advance. The irrigation schedules of pistachio, olive, grape and almond are shown in Figs. 2–5. The net irrigation water requirement (NIR) of crops is the amount of water required for crops to fully develop or to bring the decreased available soil water content in the plant root zone back to the field capacity. The NIR varies with the crop pattern and climate characteristics of the region.

Net irrigation water requirements are calculated using the following equation; (3)${\displaystyle \mathrm{NIR}={\mathrm{ET}}_{\mathrm{c}}-P_{\mathrm{eff}}}$ where; ET_c is the crop evapotranspiration, and P_eff is the effective rainfall.

The CROPWAT model uses the water budget technique detailed by Allen et al. (1998) to determine irrigation water demand (IR). For this purpose, the following equation (Eq. 4) is used in IR calculation. (4)${\displaystyle {\mathrm{D}}_{\mathrm{c}}={\mathrm{D}}_{\mathrm{p}}+{\mathrm{ET}}_{\mathrm{c}}-\mathrm{P}-\mathrm{I}+\mathrm{SRO}+\mathrm{DP}.}$

where: D_c: current day’s soil water deficit (net irrigation requirement) in the root zone; D_p: previous day soil water deficit; ET_c: current day crop evatranspiration rate; P: actual gross precipitation; I: amount of final irrigation injected into the field today; SRO: water loss by runoff from the soil surface, DP: deep percolation (Sharma & Tare, 2021).

The amount of water to be diverted from the source to the irrigation area is calculated by the ratio of NIR to the irrigation efficiency. Total irrigation efficiency is one of the most essential factors in determining irrigation modules (flow, l/sec/ha) in irrigation projects.

During the transmission of water from the source to the plant root zone in the irrigation area, some part of water is lost through mechanisms of transmission losses, deep percolation, infiltration and surface runoff. In addition to the aforementioned losses, some of irrigation water is needed for soil leaching, sowing and planting. Therefore, water losses must be taken into account and the total irrigation efficiency must be calculated to apply the amount of net irrigation water to the root zone and to bring the water content to the field capacity.

In the irrigation process, there are some definitions like RAM&TAM readily availably moisture (RAM) and total available moisture (TAM), etc. RAM is the amount of water that plants can easily take in the root zone without encountering any water stress. TAM expresses the total amount of moisture contained in the soil around plant root zone. The RAM value is also defined as a certain percentage of the TAM.

Therefore, in irrigation applications, water reductions in the root zone are allowed until the RAM level, and after this point, irrigation is started and recycled until the root zone moisture level reaches to the field capacity. TAM is also defined as the difference between soil moisture content (SMC) at field capacity and permanent wilting point (PWP) by Kanber (2015).

Effective rainfall (P_eff)

In arid or semi-arid climatic conditions, agricultural production is highly dependent on the rainfall. However, the full amount of rainfall is not always usefull for crops use. Some of rainfall is lost through deep infiltration and surface flow. Moseki, Murray-Hudson & Kashe (2019) explains that as the amount of water available to plants after all the losses have taken place is referred to as the effective rainfall. The estimation of effective rainfall values depend on the location of the study area and the amount of rainfall occurs in the region. The Eqs. (5) and (6) below, recommended for CROPWAT 8.0 by the US Soil Conservation Service, were used to determine the monthly effective rainfall values (Balasaheb & Sudarsan Biswal, 2020).

(5)${\displaystyle P_{\mathrm{eff}}=\frac{P\ast \left(125-0.2\ast P\right)}{125}\mathrm{for}P\le 250\mathrm{mm}}$ (6)${\displaystyle P_{\mathrm{eff}}=125+0.1\ast P\mathrm{for}P>250\mathrm{mm}.}$

where, Peff is the effective rainfall in mm, and P is the total rainfall in mm.

Effective rainfall values are calculated by using Eqs. (5) and (6) based on the total rainfall values. In cases or periods where effective rainfall values are higher than the ET_c values, there is no need for any irrigation. Therefore, CWR values are calculated based on the cases where ET_c values are higher than effective rainfall values.