Baseflow Simulation Using SWAT Model in an Inland River Basin in Tianshan Mountains, Northwest China

Baseflow is a streamflow component which reacts slowly to rainfall and is usually associated with water discharged from groundwater storage. Knowledge about baseflow is useful in assessing water quality, forecasting streamflow, allocating water supply, and designing hydropower plants under low-flow conditions. Baseflow is also an important component in hydrological simulation and itself may be composed of a number of components, which may vary seasonally with different recession constants.

The Soil and Water Assessment Tool (SWAT) uses a conceptual linear one-reservoir (shallow aquifer storage) approach to simulate baseflow. SWAT partitions groundwater into two aquifer systems: a shallow aquifer and a deep aquifer. While the shallow aquifer-baseflow was properly reproduced by SWAT, weaker simulation of baseflow was found as well. For example, LUO Yi et al. found underestimation of baseflow during the low-flow period in the Manas River Basin in northern Tianshan Mountains using SWAT2005.

In order to simulate the snow and/or glacier melt dominated streamflow receding quickly during the high-flow period but very slowly during the low-flow period in inland rivers in arid and cold northwest China, LUO Yi et al. developed a two-reservoir approach for baseflow simulation in SWAT by adding a slow- reacting reservoir and applying it to the Manas River basin in the Tianshan Mountains. Meanwhile, a digital filter program was employed to separate baseflow from streamflow records for comparisons.

The results indicated that the two-reservoir method yielded much better results than the one-reservoir one in reproducing streamflow processes, and the low-flow estimation was improved markedly. Nash-Sutcliff efficiency values at the calibration and validation stages are 0.68 and 0.62 for the one-reservoir case, and 0.76 and 0.69 for the two-reservoir case. The filter-based method estimated the baseflow index as 0.60, while the model-based as 0.45. The filter-based baseflow responded almost immediately to surface runoff occurrence at onset of rising limb, while the model-based responded with a delay. In consideration of watershed surface storage retention and soil freezing/thawing effects on infiltration and recharge during initial snowmelt season, a delay response is considered to be more reasonable. However, a more detailed description of freezing/thawing processes should be included in soil modules so as to determine recharge to aquifer during these processes, and thus an accurate onset point of rising limb of the simulated baseflow.

The result has been published on Hydrology and Earth System Sciences, 2012, 16: 1259-1267. The paper is also archived at http://www.hydrol-earth-syst-sci-discuss.net/8/10397/2011/hessd-8-10397-2011.html.