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Application of integrated hydrological models to manage a watershed's water resources are increasingly finding their way into the decision-making processes. The Soil and Water Assessment Tool (SWAT) is a multi-process model integrating hydrology, ecology, agriculture, and water quality. SWAT is a continuation of nearly 40 years of modeling efforts conducted by the United States Department of Agriculture (USDA) Agricultural Research Service (ARS). A large number of SWAT-related papers have appeared in ISI journals, building a world-wide consensus around the model's stability and usefulness. The current issue is a collection of the latest research using SWAT as the modeling tool. Most models must undergo calibration/validation and uncertainty analysis. Unfortunately, these sciences are not formal subjects of teaching in most universities and the students are often left to their own resources to calibrate their model. In this paper, we focus on calibration and uncertainty analysis highlighting some serious issues in the calibration of distributed models. A protocol for calibration is also highlighted to guide the users to obtain better modeling results. Finally, a summary of the papers published in this special issue is provided in the Appendix.

Iran is experiencing unprecedented climate-related problems such as drying of lakes and rivers, dust storms, record-breaking temperatures, droughts, and floods. Here, we use the ensemble of five high-resolution climate models to project maximum and minimum temperatures and rainfall distribution, calculate occurrences of extreme temperatures (temperatures above and below the historical 95th and 5th percentiles, respectively), analyze compound of precipitation and temperature extremes, and determine flooding frequencies across the country. We found that compared to the period of 1980–2004, in the period of 2025–2049, Iran is likely to experience more extended periods of extreme maximum temperatures in the southern part of the country, more extended periods of dry (for ≥120 days: precipitation <2 mm, Tmax ≥30 °C) as well as wet (for ≤3 days: total precipitation ≥110 mm) conditions, and higher frequency of floods. Overall, the combination of these results projects a climate of extended dry periods interrupted by intermittent heavy rainfalls, which is a recipe for increasing the chances of floods. Without thoughtful adaptability measures, some parts of the country may face limited habitability in the future.

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Water resources availability in the semiarid regions of Iran has experienced severe reduction because of increasing water use and lengthening of dry periods. To better manage this resource, we investigated the impact of climate change on water resources and wheat yield in the Karkheh River Basin (KRB) in the semiarid region of Iran. Future climate scenarios for 2020–2040 were generated from the Canadian Global Coupled Model for scenarios A1B, B1 and A2. We constructed a hydrological model of KRB using the Soil and Water Assessment Tool to project water resources availability. Blue and green water components were modeled with uncertainty ranges for both historic and future data. The Sequential Uncertainty Fitting Version 2 was used with parallel processing option to calibrate the model based on river discharge and wheat yield. Furthermore, a newly developed program called critical continuous day calculator was used to determine the frequency and length of critical periods for precipitation, maximum temperature and soil moisture. We found that in the northern part of KRB, freshwater availability will increase from 1716 to 2670 m3/capita/year despite an increase of 28% in the population in 2025 in the B1 scenario. In the southern part, where much of the agricultural lands are located, the freshwater availability will on the average decrease by 44%. The long‐term average irrigated wheat yield, however, will increase in the south by 1.2%–21% in different subbasins; but for rain‐fed wheat, this variation is from −4% to 38%. The results of critical continuous day calculator showed an increase of up to 25% in both frequency and length of dry periods in south Karkheh, whereas increasing flood events could be expected in the northern and western parts of the region. In general, there is variability in the impact of climate change in the region where some areas will experience net negative whereas other areas will experience a net positive impact. Copyright © 2013 John Wiley & Sons, Ltd.

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Large-scale distributed watershed models are data-intensive, and preparing them consumes most of the research resources. We prepared high-resolution global databases of soil, landuse, actual evapotranspiration (AET), and historical and future weather databases that could serve as standard inputs in Soil and Water Assessment Tool (SWAT) models. The data include two global soil maps and their associated databases calculated with a large number of pedotransfer functions, two landuse maps and their correspondence with SWAT’s database, historical and future daily temperature and precipitation data from five IPCC models with four scenarios; and finally, global monthly AET data. Weather data are 0.5° global grids text-formatted for direct use in SWAT models. The AET data is formatted for use in SWAT-CUP (SWAT Calibration Uncertainty Procedures) for calibration of SWAT models. The use of these global databases for SWAT models can speed up the model building by 75–80% and are extremely valuable in areas with limited or no physical data. Furthermore, they can facilitate the comparison of model results in different parts of the world.

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