Predicting Water Quality Assessments in Agricultural Fields of the Mississippi Delta.

The lack of information in many regions regarding watershed monitoring for water quality and quantity and their associated costs has promoted the frequent use of analytical tools to explore actions and policy alternatives for managing both, water quality and quantity from intensive agricultural fields. The evaluation and selection of appropriate applicable analytical tools has been identified as one of the strategies needed for designing, siting and assessing the potential reductions from multiple management practices implemented within the Mississippi Delta region. The present study is a component of the activities oriented to the evaluation and selection of those appropriate analytical tools. The study was conducted i) to predict the effect of changing land use and management practice scenarios on runoff quantity/quality and crop productivity from an 11.3 ha agricultural subwatershed in the Mississippi Delta region; and ii) to assess the potential for phosphorus runoff under the simulated scenarios. The Agricultural Policy Environmental Extender (APEX) model was used to predict changes on runoff quality and quantity, while the MS Phosphorus Index was the tool used to complete the risk assessment analysis. APEX model was calibrated and validated by comparing predicted results over four years of monitoring information for daily, monthly and annual runoff depths, sediment and phosphorus loads, soil properties and crop productivity changes under reduced tillage practices and establishment of cover crops after the crop season. Water quality and quantity changes were predicted from additional land management scenarios that were setup on APEX (conventional tillage with and without cover crops, reduced tillage with cover crops, establishment of different crops, and nutrient management). Predictions were used to rank the relative risk of surface water contamination resulting from phosphorus for each one of the evaluated scenarios. Results showed that APEX predicted reasonable magnitudes that are comparable with reported values on losses of sediments and phosphorus from evaluated scenarios, condition that makes APEX to be satisfactorily considered as an appropriate tool at field and watershed scales to develop action plans to enhance the nutrient reduction strategy within the Mississippi Delta region. The MS Phosphorus Index was no sensitive at all to any potential change on water quality and a further sensitivity analysis needs to be performed to identify the importance of individual input factors on final P index rating.