Introducing and evaluation of relative economic water productivity index
In order to evaluate the result of any activity, a comprehensive and complete index is required to take into account all factors affecting the activity. In the field of agriculture, an index called water productivity is used for this purpose in different ways. Performance index, Benefit-Cost Ratio (B/C), Physical Water Productivity index (WPPa), Economic Water Productivity index (WPEa), Physical Productivity Gap index (WPGp) and Economic Productivity Gap index (WPGe) are used to evaluate the performance of farmers and gardeners. The WPPa index does not consider product price and costs and therefore cannot be a suitable indicator for choosing a plant for the farm or garden. The WPEa index cannot provide an accurate assessment of various crops/plants due to the influence of inflation and market regulation policy and the absence of the soil and climate properties of the regions. The water productivity gap index (WPGp & WPGe) cannot provide a correct assessment of water productivity due to the different nature of yield and price of different plants.
Therefore, it is necessary to introduce an index that firstly takes into account all factors affecting agricultural production and secondly does not get affected by factors ineffective on production, such as inflation and market regulation policies, and provides an accurate assessment of various field and garden products.
In the present study, the relative economic water productivity index (KWPe) was introduced as the ratio of economic water productivity in real conditions to economic water productivity in standard conditions (potential). This index was evaluated for different factors affecting production, including soil, plants, climate, management and inflation and compared with the traditional physical and economic water productivity and productivity gap indicators. In this research, The relative physical water productivity index (KWPp) was also calculated and compared with the KWPe. To calculate economic water productivity under potential conditions, net crop water requirement was calculated using the Penman-Monteith, and potential yield determined based on the land potential production, which depends on the soil and climate of the region.
The results showed that all factors affecting agricultural production have a logical effect on the KWPe index. But the WPPa, WPEa, KWPp, WPGe and WPGp indices are either not affected by all the factors affecting production, or they are affected by factors other than the factors affecting the reproduction of inflation and market regulations. For this reason, by using the proposed KWPe index, it is possible to assesst the crop pattern (what to plant) and the combination of cultivation (at what level to plant) with higher accuracy. Due to the fact that in the KWPe index, the water productivity of each plant is measured based on the standard conditions of each plant, the productivity of several crops in one region or a single plant in several regions can be compared using the introduced index. As the price and cost is not considered in KWPp index ca;culation, it is less accurate than the KWPe index in providing water productivity in different plants. In addition, in the KWPe, WPGe and WPGp indices, the soil and climate of the region are directly included in the water productivity index through the net water requirement and land production potential. But the soil and climate of the region are not considered in WPPa and WPEa indices. Unlike other indices, the KWPe index was not affected by the five times price of tomatoes, and introduced soybean plant as the most productive plant with a relative productivity of 0.86. Despite the equality of water productivity gap for wheat and barley (3973 toman/m3), in the same conditions, the KWPe index was estimated to be 0.67 for wheat and 0.33 for barley. In briefly in terms of accuracy, the order of proposed indicators are the WPPa, WPEa, WPGp, WPGe, KWPp and KWPe is recommended to evaluate water productivity in different plants, climates, soils, managements and inflation.