Using the SWAT Model in Intensively Managed Irrigated Watersheds: Model Modification and Application

The Soil and Water Assessment Tool (SWAT) is a well-established modeling tool for simulating hydrologic processes in watershed systems. However, SWAT often does not perform well in highly managed watersheds with intensive irrigation due to management practices and associated hydrologic processes (e.g., canal seepage) not included in the modeling code. This study presents a method to apply SWAT to managed irrigated watersheds, which includes (1) designating each cultivated field as a hydrologic response unit (HRU) and including recorded crop rotations; (2) applying scheduled irrigation according to water rights; and (3) simulating seepage from earthen irrigation canals, with the latter requiring a minor modification to the SWAT code. SWAT’s autoirrigation function is also used as a comparison. The methodology was applied to a $732\text{-}{\mathrm{km}}^2$ watershed in the Lower Arkansas River Valley (southeastern Colorado), a semiarid region that has been intensively irrigated for over 100 years. The model was tested against monthly stream discharge at five stream gauging stations in the Arkansas River and its tributaries during the 2001–2007 period. Results demonstrate the necessity of including human management features, particularly canal seepage, to capture in-stream flows in the river and tributaries. The scheduled irrigation approach has a significantly better performance in the smaller highly agricultural tributaries whereas the autoirrigation functionality has a slightly but not significantly better performance at the outlet of the whole watershed. The methods presented herein can be applied to other semiarid irrigated regions.