Quantification and simulation of surface runoff from fescue grassland watersheds

The topographic features of the foothills fescue grasslands in southern Alberta predispose them to runoff and soil loss via erosion. A study was conducted at Stavely Research Station, Alberta to determine the runoff from small grassland watersheds under three grazing intensities, viz. ungrazed (or control), heavy (2.4 animal unit months per hectare (AUM ha(-1))) and very heavy (4.8 AUM ha(-1)) grazing. Total annual precipitation in 1998, 1999 and 2000 was 648, 39 and 263 mm, respectively. Surface runoff hydrographs indicated large summer storm runoff rates from heavy grazed compared to other watersheds, but large snow melt-induced runoff from very heavy grazed compared to other watersheds. Surface runoff rates measured from May and August ranged between 0 and 2.3 mm per day in 1998, 0-0.2 mm per day in 1999, and 0-0.07 mm per day in 2000. In all the years, the average rainfall runoff was <10% of average daily precipitation on all three watersheds. In 2000, snow melt-induced runoff was measured in March. Total surface runoff for this month was 0.07, 8.5 and 3.7 mm for ungrazed, heavy and very heavy watersheds, respectively. These accounted for 78, 96 and 92% of total annual runoff from ungrazed, heavy and very heavy watersheds, respectively. Surface runoff for 1999 and 2000 was simulated using Soil Water Assessment Tool (SWAT), a continuous time distributed parameter model developed for ungaged basins. Model calibration was conducted using data of 1998 and parameters adjusted until the predicted and observed results were visibly close. Evaluation of the model was conducted using statistical criteria that included calculations of average error (A-E), residual mean square (RMS), coefficient of residual mass (CRM) and modeling efficiency (EF), and comparing these statistics against optimal values. The evaluation indicated that the model under-predicted surface runoff from the watersheds in both the years. (C) 2002 Published by Elsevier Science B.V.