WaterSubject

Borkhar district is located in an arid to semi-arid region in Iran and regularly faces widespread drought. Given current water scarcity, the limited available water should be used as efficient and productive as possible. To explore on-farm strategies which result in higher economic gains and water productivity (WP), a physically based agrohydrological model, Soil Water Atmosphere Plant (SWAP), was calibrated and validated using intensive measured data at eight selected farmer fields (wheat, fodder maize, sunflower and sugar beet) in the Borkhar district, Iran during the agricultural year 2004–2005. The WP values for the main crops were computed using the SWAP simulated water balance components, i.e. transpiration T, evapotranspiration ET, irrigation I, and the marketable yield Y_M in terms in terms of Y_MT^?1, Y_M ET^?1 and Y_M I^?1.

The average WP, expressed as $ T^?1 (US $ m^?3) was 0.19 for wheat, 0.5 for fodder maize, 0.06 for sunflower and 0.38 for sugar beet. This indicated that fodder maize provides the highest economic benefit in the Borkhar irrigation district. Soil evaporation caused the average WP values, expressed as Y_M ET^?1 (kg m^?3), to be significantly lower than the average WP, expressed as Y_M T^?1, i.e. about 27% for wheat, 11% for fodder maize, 12% for sunflower and 0.18 for sugar beet. Furthermore, due to percolation from root zone and stored moisture content in the root zone, the average WP values, expressed as Y_M I^?1 (kg m^?3), had a 24–42% reduction as compared with WP, expressed as Y_M ET^?1.

The results indicated that during the limited water supply period, on-farm strategies like deficit irrigation scheduling and reduction of the cultivated area can result in higher economic gains. Improved irrigation practices in terms of irrigation timing and amount, increased WP in terms of Y_M I^?1 (kg m^?3) by a factor of 1.5 for wheat and maize, 1.3 for sunflower and 1.1 for sugar beet. Under water shortage conditions, reduction of the cultivated area yielded higher water productivity values as compared to deficit irrigation.

P indicators express the benefit derived from the consumption of water and can be used for assessing the impact of on-farm strategies under water scarce conditions. They provide a proper vision of where and when water could be saved. WP indicators are also useful for looking at the potential increase in crop yield that may result from increased water availability.

Quantitative information on WP indicators is therefore necessary to plan an efficient irrigation water management under water scarce conditions. In order to explore which farm strategies help us to achieve ‘more crop per drop’, we need to understand the interactions between soil, atmosphere, crop and water. Taking in account the spatial variability of the soil and the land use properties, as well as crop growth development, simulation of the water balance components will certainly increase our ability to improve water productivity under water shortage conditions.

In the past 30 years, physically based agrohydrological models such as the Soil Water Atmosphere Plant (SWAP) model  have been developed to simulate crop growth and soil water processes. Simulation models are strong in scenario analyses and thus permit to explore viable ways of crop and water management which may help to mitigate the impact of drought. However, agrohydrological models use a large number of input parameters which may cause low performance and large uncertainty in simulation results such as actual evapotranspiration, deep percolation and dry matter yield.

In this paper, a methodology will be presented for evaluation of on-farm strategies under water-limited conditions. The SWAP model will be calibrated and validated for the main crops in the Borkhar irrigation district using measurements from farmer’s fields. Most of the input parameters collected from these fields will be used directly in the calibration procedures. The remaining unknown soil hydraulic parameters, irrigation depths and crop parameters will be determined indirectly by an inverse modelling technique.

With the help of automated runs using the link between SWAP and the Parameter ESTimation model PEST, WP-water consumption curves will be presented for the main crops and for different irrigation practices. On-farm strategies like deficit irrigation scheduling and cropped area reduction will be evaluated using WP indicators. The maximum possible increases in WP indicators will be derived graphically from the WP curves.

This research in Borkhar district (Isfahan), Iran, was financed by the Iranian Ministry of Science, Research and Technology and was supported by the Agricultural Planning and Economic Research Institute in Iran, Iranian Soil and Water Research Institute in Esfahan. The authors would like to thank Prof. A. Alizadeh (Professor of Irrigation Group at Ferdowsi University of Mashad, Iran), Dr. A. Sharifi (Associate professor at Department of Urban-Regional Planning and Geo-Information Management, International Institute for Geo-Information Science and Earth Observation, ITC, The Netherlands), M. Fathi (Researcher at the Soil and Water Research Institute in Esfahan, Iran) for his help during early phases of this research. The authors thank also the anonymous reviewers for their constructive comments.