Effect of water price under different allocation scenarios on crop pattern economic productivity

Introduction
Crop water productivity (CWP) is defined as the crop yield produced per unit of water consumed, which can be improved by increasing the crop yield with a given water usage or reducing water usage with a given yield. Increasing CWP can thus help to alleviate the water crisis while ensuring food security. Physical productivity alone is not enough to determine the crop pattern and economic productivity should also be considered. Economic water productivity (EWP) expressed as the gross income in USS per unit of water consumed, is relevant for farmers to pursue higher net benefits. Both CWP and EWP terms are important indices for water resource managers to consider when formulating planning policies. The simultaneous consideration of CWP and EWP allows for a more comprehensive and robust exploration when planning the process for developing regional agricultural water-saving measures, such as modifying the regional cropping distribution. This allows farmers to reduce irrigation water use and shift the area of water-intensive crops to ones with efficient water use or higher economic value. Determining crop pattern-based water productivity is especially important in countries with dry climates, whose agriculture depends solely on irrigation and also has low water consumption efficiency. Therefore, instead of consuming scarce water resources, in the production of products that consume a lot of water, it is possible to produce products with lower water consumption and avoid excessive pressure on water resources. Knowledge of crop-water requirements is crucial for water resources management and planning to improve water-use efficiency. Crop water requirements in the growing period depends on the crop growth stage, cropping technique, and irrigation method. About 99 % of the water uptake by plants from soil is lost as evapotranspiration (ET), so, it can be stated that the measurement of actual crop evapotranspiration (ETc) on a daily scale for the whole vegetative cycle is equal to the water requirement of the given crop. Evapotranspiration is defined as the water lost as vapor by an unsaturated vegetative surface and it is the sum of evaporation from soil and transpiration by plants. Knowledge of the exact water loss through actual evapotranspiration is necessary for accurate and effective water management.
 
Materials and Methods
For this purpose, in the first stage agricultural condition of the aquifer was investigated through a questionnaire by farmers and experts. To calculate the reference crop evapotranspiration we used the Penman-Monteith equation in this study crop coefficient curves have been prepared according to the agricultural calendar of the Basht aquifer. Net water requirement is calculated from the difference between effective rainfall and evapotranspiration. Water productivity per crop ( ) (kg.m^-3) is an important index for determining the agricultural production system efficiency. Water productivity is defined as the proportion of crop yield (kg.ha^-1) to irrigation water consumed by crops in the field (m^-3.ha^-1). Likewise, water economic productivity is measured about or with the economic benefits in a monetary value of outputs over the number of necessary inputs such as water depleted. To calculate the value of each cubic meter of water, the production costs (minus the water) need to be deducted from the income and the remainder needs to be divided by the volume of water. The calculation results are calculated separately for each product. To determine the suitable pattern crop in Basht aquifer, different cropping patterns were evaluated (eight different scenarios).
 
Results and Discussion
Based on the results of the Penman-Monteith method, it can be concluded that the gross water requirement (the amount of net irrigation requirement divided by the irrigation efficiency) in dominant crops of aquifer including rice, alfalfa, citrus, watermelon, corn, wheat, rapeseed, legumes, barley respectively were 20234, 14083, 9291, 9170, 7863, 5630, 5411, 5225, and 4821 m^-3 ha, respectively. The amount of effective precipitation that provided a part of the crop’s water requirement through soil moisture (green water) for water crops such as Rice and Corn is close to zero. Autumn crops such as canola, citrus fruits, and cereals use green water. To determine the amount of irrigation per hectare of the current crop pattern of the aquifer, the hydro module of each crop was determined. As it is clear from hydromodule, the average required water flow (l s^-1 ha^-1) for rice, alfalfa, citrus, watermelon, corn, rapeseed, wheat, beans, and barley, equaled 0.63, 0.44, 0.29, 0.29, 0.24, 0.18, 0.17, 0.16 and 0.15 (l s^-1 ha^-1) respectively. In total, the amount of water consumed by the agricultural products in the aquifer's Basht is 45 millm³, that approximately equivalent to one m³ m^-2 of the aquifer cultivation area and this amount is much more than the aquifer agriculture programmable water. The economic productivity of the aquifer’s current cultivation is 45,000 IRR m^-3, on average. Also, most aquifer products' physical productivity was less than one. the comparison of different patterns showed that scenarios eight and twohad the highest and lowest amount of water consumption, 45 and 22 millm³, respectively.
 
Conclusion
The crop pattern will be influenced by parameters such as climatic compatibility of products, water, and soil potentials, needs, interests of agriculture producers, and production income. In the Basht aquifer, the availability of water and the amount of water consumed is one of the most important factors in choosing the cultivation pattern. In the current situation, due to the high temperature and increasing evaporation rate, and the use of seasonal rainfall, crops that spend their growth period in autumn and winter should be included in the cultivation pattern. The simultaneity of water requirements for crops planted together is one of the important parameters in choosing the cultivation pattern. In the Basht aquifer, the water requirement of corn, alfalfa, cucumber, and tomatoes coincide with the citrus water requirement during the time of high water consumption, and the cultivation of one of them may create water limitations for the other crop. In contrast, the cultivation of wheat, barley, and canola have a very small overlap with the citrus irrigation times. Choosing a combination of citrus, wheat, barley, and canola will optimize the cultivation pattern.