Application of SWAT model to investigate nitrate leaching in Hamadan–Bahar Watershed, Iran
Research highlights
▶ We used the Soil and Water Assessment Tool (SWAT) to build a hydrologic model for the Hamadan–Bahar watershed in western Iran. The watershed suffers from an increasing water pollution problem due to agricultural activities. ▶ The hydrology model was calibrated, and validated with uncertainty analysis. ▶ We used the calibrated hydrology model and further calibrate it for nitrate leaching and crop yield, using wheat and potato. ▶ We characterized the spatial and temporal leaching pattern. ▶ In a follow up work we will suggest a best management practice for the region.
Introduction
In Hamadan–Bahar watershed, water scarcity has become an increasingly serious problem. Groundwater is the major source of water supply for drinking, domestic, industrial, and agricultural sectors in this region. One of the problems affecting the quality of groundwater is leaching nutrients from the soil, which is especially evident in agricultural dominated watersheds (Jalali, 2005). Application of large amounts of nitrogen fertilizers, at higher than crop uptake requirement rate, in intensive agricultural regions of Hamadan–Bahar plain contributes to excessive nitrate accumulation in soils and leaching into groundwater bodies (Jalali, 2005, Nadafian, 2007, Rahmani, 2003).
Nitrogen leaching from agricultural lands is a widespread global problem. In areas where nonpoint source pollution is dominant, regional models are often the only practical way to examine the impacts of changing landuse on the concentration of nitrate pollution. Hydrological models that are able to calculate the nitrogen fate and transport are useful tools to determine the probable effects of agricultural activities on local hydrology and aqueous geochemistry. Direct measurement of the impact of agricultural practices on groundwater quality is tedious and usually too expensive. For example, soil sampling is time consuming and sampling water quality at field level, using suction cups or lysimeters is expensive and impractical (Lord and Shepherd, 1993). Such approaches cannot directly help in making a general decision in large scales. These issues demand integrated management of water resources in watersheds (Pohlert et al., 2007), which rely mostly now on modeling.
Nonpoint source nitrate loading has substantially impacted groundwater nitrate concentrations in Hamadan–Bahar aquifer (Jalali, 2005, Nadafian, 2007, Rahmani, 2003). These water resources are located in the vicinity of drinking water wells. Therefore, it is essential to determine how management practices will impact groundwater nitrate concentrations.
With this background in mind, the objectives of this study included: (1) using the SWAT program to model the temporal and spatial variability of nitrate leaching dynamics for the present agricultural activities at hydrologic response unit (HRU) level with monthly time steps, (2) to calibrate, validate and perform uncertainty analysis for the SWAT hydrologic model for Hamadan–Bahar watershed based on river discharge and nitrate data, and then (3) to calibrate, validate and perform uncertainty analysis for the SWAT crop yield model using the main regional crops, which are potato (Solanum tuberosum L.) and irrigated and rainfed wheat (Triticum asestivum L.).
Section snippets
Description of the study area
The Hamadan–Bahar watershed with an area of 2460 km2 is situated between longitudes of 48°7′E and 48°52′E and latitudes of 34°35′N and 35°12′N, in western Iran (Fig. 1). In this watershed, most of the rivers originate from southern heights (Alvand Mountains). All of the rivers merge in the central plain and form the Siminehrud (Fig. 1). The outlet of watershed is Koshkabad in the north-east. Mean elevation of the watershed is 2038 m above mean sea level. The average daily discharge at Koshkabad
Calibration and uncertainty analysis
Table 6 has a listing of the SWAT model parameters included in the calibration process and their sensitivity statistics. The results of the sensitivity analysis indicated that parameters selected for stream flow were sensitive parameters. The t-value (Table 6) provides a measure of sensitivity (larger values are more sensitive) and p-values determine the significance of the p-value (the smaller, the more significant) (Abbaspour, 2007).
Curve number was the most sensitive parameter for stream
Conclusions
The SWAT model was successfully employed to simulate runoff, crop yield and river nitrate for Hamadan–Bahar watershed with uncertainty analysis. Two important modifications in SWAT setup were the implementation in the irrigation module to estimate percolation and the consideration of spring discharge to predict base flow. One of the difficulties and limitations within this study was the lack of data on the amount of irrigation water that is withdrawn from rivers or creeks. However given the
Acknowledgements
The authors wish to acknowledge Hamadan Regional Water Authority, Water and Wastewater Co. of Hamadan, Isfahan University of Technology, the Swiss Federal Institute for Aquatic science and Technology (Eawag), and Jahade-Agriculture of Hamadan for providing assistance to conduct this study.
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