Evaluation and Update of Kansas Phosphorus Index Using the Apex Model.

Agricultural fields contribute substantial source of phosphorus (P) to water resources. Failure to control P export from agriculture can promote algal blooms and degrade water quality. Reduction of P loss from agricultural production requires accurate estimates of how management practices impact P loss relative to soils, landscape position, slope, and hydrology. Field studies, computer models such as the Agricultural and Policy Environmental Extender (APEX) and phosphorus index (PI) can be used as tools to assess the impact of management practices on P loss. Field-scale studies to evaluate P loss are expensive, time consuming and results are highly dependent on the weather patterns. Processed-based computer models offer an alternative but are very difficult to use and have not been adequately tested. And there are concerns about the accuracy of P indices achieving water quality goals and ranking the risk of P loss. Therefore, current methods must be improved and PI needs to be evaluated to better estimates and minimize P loss from agricultural fields. The objective of the study was to evaluate and update Kansas PI using the model simulated results. Five treatments in a Grain Sorghum -Grain Sorghum rotation (2011-2013), Crawford, KS and five treatments in a Grain Sorghum-Soybean rotation (2001- 2004), Franklin, KS runoff study were used to calibrate and validate the APEX model for runoff, sediment, and TP loss on event basis. The APEX model performance was evaluated using the Nash Sutcliffe efficiency (NSE), coefficient of determination (r²), and percent bias (P-bias). The fully calibrated and validated model will be used to simulate the effect of best management practices (BMPs). The model simulated results then will be used to evaluate and update Kansas PI.