No abstract

Forest clear cutting alters the hydrological processes such as interception, evapotranspiration and infiltration of the forested watershed and consequently increases the amount of water and sediment leaving the watershed. This study was conducted in the Upper Pearl River Watershed (UPRW) located in east-central Mississippito evaluate and compare the potential impacts of forest clear cutting on water and sediment yields using the Soil and Water Assessment Tool (SWAT) model. For this purpose, five hypothetical scenarios representing clear cutting at 10%, 20%, 30%, 55% and 75% of the total forest area of the watershed were generated. The SWAT model was first calibrated (1981-1995) and validated (1996-2008) for monthly stream flow, and later verified (February 2010 to December 2010) for monthly sediment load. Results show that the SWAT model was able to simulate stream flow and sediment load satisfactorily during the calibration/validation and verification periods, respectively. The potential changes caused in yields as a result of the changes in clearcut area were computed by comparing predicted yields from each clear cutting scenario and a base condition. Results from five scenarios demonstrate substantial increase in yields with an increase in the percentage of forest area clearcut. When compared with the base scenario, potential changes in water and sediment yields occur between 17% to 96% and 33% to 250%, respectively, with an increase in clearcut area from 10% to 75%. Results also indicate that, for all scenarios, percentage wise change is larger for sediment yield. Although predicted water and sediment yields generated from each scenario are subject to further verification with observed data, this study provides useful information about the potential amount of water and sediment yields that may be produced under each scenario, and that the potential changes that may happen on yields if similar magnitude of clear cutting occurs in the UPRW or in similar watershed.

Sensitivity analysis of crop parameters and the performance of SWAT (Soil and Water Assessment Tool) model to simulate potential forest biomass production were evaluated for the Upper Pearl River Watershed (UPRW). Local sensitivity analysis of seven crop parameters: radiation use efficiency (kg/ha)/(MJ/m 2) (BIOE), potential maximum leaf area index for the plant (BLAI), fraction of growing season at which senescence becomes the dominant growth process (DLAI), fraction of the maximum plant leaf area index corresponding to the 1st point on the optimal leaf area development curve (LAIMX1), fraction of growing season corresponding to the 1st point on the optimal leaf area development curve (FRGRW1), plants potential maximum canopy height (m) (CHTMX), and maximum rooting depth for plant (mm) (RDMX) reveals that only three parameters: DLAI, BIOE and BLAI are sensitive to forest biomass production. Further, results indicate moderate sensitivity of DLAI and BIOE and low sensitivity of BLAI with relative sensitivity index of 0.44, 0.35 and 0.14, respectively. The performance of SWAT to simulate potential forest biomass was evaluated by comparing simulated data against three years of observed data that were obtained from USDA Forest Service website. The results indicate satisfactory performance of SWAT in predicting potential forest biomass, which is shown by the high value of coefficient of determination (R 2 = 0.83), small root mean square error (RMSE = 11.11 Mg/ha), and small difference between mean. Results also reveal that the UPRW has the potential to produce approximately 49 Mg/ha of average forest biomass annually, which is approximately 6% less than the observed biomass.

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