Calibration and Validation of SWAP to Simulate Conjunctive Use of Fresh and Saline Irrigation Waters in Semi-Arid Regions Ajit Kumar Verma & Suresh Kumar Gupta & Rajendra Kumar Isaac Received: 14 October 2012 / Accepted: 9 July 2013 # Springer Science+Business Media Dordrecht 2013 Abstract Soil–Water–Atmosphere–Plant (SWAP) version 2.0 was evaluated for its capability to simulate crop growth and salinity profiles at Agra (India) located in a semi-arid region having deep water table and monsoon climate. The data of 12 conjunctive use treatment combinations simulat- ing cyclic and mixing modes of fresh and saline water for wheat were used to calibrate and validate the model. Absolute deviations between the SWAP simulated and ob- served relative yields during calibration ranged from 2.5 to 2.9 %. A close agreement in the trend and values of mea- sured and simulated soil salinity profiles was observed. Scenario building simulations carried out with the validated SWAP revealed that the maximum crop yields varied from 97 to 99 % with the best available water (EC 3.6 dS m -1 ) while the minimum ranged from 65 to 79 % in the treatment with all saline water. Other than this, the relative yield varied from 80 to 98 % in 10 other cyclic and mixing mode treat- ments. It was established that notwithstanding the seasonal build-up of salts due to saline water use, there would be no long-term build-up of salts as leaching during the monsoon season would render the soil profile salt free at the time of sowing of rabi (winter) crops. Thus, short-term field obser- vations could be used in conjunction with SWAP to show that there seems to be an assured long-term sustainability when saline water is used in conjunctive mode with fresh water in monsoon climatic conditions with deep water table. These results are in conformity with the observation that many farmers in India are using saline and fresh water in conjunctive mode on a long-term basis. Keywords Simulation . SWAP . Cyclic and mixing mode . Relative yield . Salinity profile . Wheat 1 Introduction In many arid and semi-arid regions, irrigation water supplies through the canal systems are limited. The ground water quality is also unfavorable which poses a threat to the sus- tainability of irrigated agriculture. On the other hand, use of poor-quality ground water could meet the shortfall in sup- plies and even release high-quality water for other sectors of economy to meet their increasing demand. Use of saline water for irrigation has received considerable attention in this region [7, 28–30]. Field experiments on short-term basis have proved the potential of saline water use for crop pro- duction [8, 11, 20, 22, 27, 34]. Various strategies have been proposed to use the saline waters. Cyclic and mixing use strategies have advantage in most cases and could be a practical solution to fully meet water requirement where non-saline water is limited [3, 24]. A conventional way to evaluate management options to use saline water is through conducting field trials. Although many researchers have conducted experiments on cyclic and mixing modes, most of these are for limited time periods. To arrive at sound conclusions, one needs to conduct long-term experiments. If so, the conventional techniques would re- quire huge manpower, financial resources, and other infra- structure. Another approach is to utilize the limited field experimental data and using appropriate mathematical models to arrive at the most appropriate option(s). It seems that simulation/computer models offer excellent support to reduce the required time and effort on repeat field testing A. K. Verma (*) Central Institute of Fisheries Education, Panch Marg, Yari Road, Versova, Andheri (W), Mumbai 400 061, India e-mail: akverma45@yahoo.com S. K. Gupta Central Soil Salinity Research Institute, Karnal 132001, India R. K. Isaac Sam Higginbottom Institute of Agriculture, Technology and Sciences (SHIATS), Allahabad 211007, India Environ Model Assess DOI 10.1007/s10666-013-9379-x