Submitted to: Watershed Management Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/1/2005
Publication Date: 3/1/2005
Citation: Gowda, P., Mulla, D.J. 2005. Scale effects of STATSGO vs. SSURGO soil databases on water quality predictions. In: Proceedings of the American Society of Agricultural Engineers Watershed Management to Meet Water Quality Standards and Emerging TMDL Conference, March 5-9, 2005, Atlanta, Georgia. p. 579-587. Interpretive Summary: In the United States, State Soil Geographic (STATSGO, 1:250,000 scale) and Soil Survey Geographic (SSURGO, 1:24,000 scale) are the two most commonly used spatial soil databases. They are used for assessing impacts of existing and alternative agricultural management practices on water quality and to develop Total Maximum Daily Loads (TMDL) for impaired watersheds. So far, little is known on the effect of spatial scale (1:250,000 vs. 1:24,000) on water quality predictions. Further, the amount of time and resources needed to use SSURGO is significantly higher than that for STATSGO. We conducted a study to quantify the effect of scale on sediment, nitrate and phosphorus losses from an agricultural watershed in south-central Minnesota. Calibrated model results were in excellent agreement with observed data irrespective of the soil databases used to derive soil information. However, evaluation of alternative management practices indicated that STATSGO based nitrate loss predictions were consistently higher than that for SSURGO data and vice-versa for phosphorus loss predictions. This brings up an important issue in developing TMDLs for impaired watersheds where stakeholders with conflicting interests may opt for soil databases that support their interests.
Technical Abstract: Soil information is one of the crucial inputs needed to assess the impacts of existing and alternative agricultural management practices on water quality. Therefore, it is import to understand the effects of spatial scale at which soil database is developed. In the United States, STATSGO and SSURGO are most commonly available soil databases. This study attempts to quantify the effect of scale by employing STATSGO (1:250,000) and SSURGO (1:24,000) soil databases by predicting and comparing flow, sediment, nitrate and phosphorus losses for High Island Creek, a minor agricultural watershed located in south-central Minnesota. For this purpose, a water quality model was calibrated for flow, sediment, nitrate and phosphorus losses for two years (2001-2002) using STATSGO and SSURGO soil databases. Further, the calibrated model was used to evaluate alternative tillage and fertilizer management practices such as adoption of rate of conservation tillage, rate, timing and method of N- and P-fertilizer applications. Statistical comparison of calibration results with observed data indicated excellent agreement for both (STATSGO with r2 of 0.95, 0.97, 0.77 and 0.92 and SSURGO with r2 of 0.90, 0.97, 0.82 and 0.99 for flow, sediment, nitrate and phosphorus losses, respectively) soil databases. However, evaluation of alternative management practices indicated that STATSGO based predicted annual nitrate losses are consistently higher than that for SSURGO data and vice-versa for predicted phosphorus losses. This brings up an important issue in developing TMDLs for impaired watersheds where conflicting interests of stakeholders may opt for soil database that support their interests.