Evaluation of the AnnAGNPS model for predicting runoff and sediment yield in a small Mediterranean agricultural watershed in Navarre (Spain)

Authors: CHAHOR, YOUSSEF - Public University Of Navarra; CASALÍ, JAVIER - Public University Of Navarra; GIMÉNEZ, RAFAEL - Public University Of Navarra; Bingner, Ronald - Ron; CAMPO, M - Public University Of Navarra; GONI, M - Public University Of Navarra

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2013
Publication Date: 1/1/2014
Publication URL: http://handle.nal.usda.gov/10113/58223
Citation: Chahor, Y., Casalí, J., Giménez, R., Bingner, R.L., Campo, M.A., Goni, M. 2014. Evaluation of the AnnAGNPS model for predicting runoff and sediment yield in a small Mediterranean agricultural watershed in Navarre (Spain). Agricultural Water Management. 134:24-37. http://dx.doi.org/10.1016/j.agwat.2013.11.014.

Interpretive Summary: In Spain, few studies have been carried out to explore the impact of agricultural practices on runoff and sediment load. Most of the available studies have evaluated erosion at plot scale. A study was developed to evaluate the capabilities of the US Department of Agriculture AnnAGNPS watershed pollutant model on event, monthly, and annual scales providing suitable adjustments of runoff and sediment loads in a small homogeneous Mediterranean agricultural watershed producing winter cereals (wheat and barley) located in the Navarre region of Spain using nine years of continuous data monitoring. Data for the first five years (2003-2007) were used for calibration and the remaining four years (2008 - 2011) for validation. Calibration for runoff was performed by modifying SCS Curve Number (CN) values for different stages of the main crops. Results for the surface runoff at annual, monthly and seasonal scales both in calibration and in validation processes were satisfactorily simulated. A sensitivity analysis was carried out to evaluate the sensitivity of seven input parameters for sediment load prediction. Results indicated the model was capable of simulating sediment load at an annual scale with a difference of less than 1% for calibration and 7% for validation. Although, simulation results at monthly and seasonal scales were less precise. The model response can be considered as satisfactory for determining the long term impact of conservation management practices and for risk & cost/benefit analyses. The use of stream network and corridor models, available within the suite of tools used with AnnAGNPS that include more detailed science for the channel hydraulics, morphology, and transport of sediments, would provide for a better account of sediment loads at event, seasonal and monthly time scales, for this small watershed.

Technical Abstract: AnnAGNPS (Annualized Agricultural Non-Point Source Pollution Model) is a system of computer models developed to predict non-point source pollutant loadings within agricultural watersheds. It contains a daily time step distributed parameter continuous simulation surface runoff model designed to assist with determining BMPs, the setting of TMDLs, and for risk & cost/benefit analyses. The purpose of this study was to evaluate the capabilities of the model to simulate runoff and sediment loads in a small Mediterranean agricultural watershed located in the region of Navarre (Spain) using nine years of continuous data monitoring. Data for the first five years (2003-2007) were used for calibration and the remaining four years (2008 - 2011) for validation. No significant channel erosion nor in-stream structures were identified within this small and homogeneous watershed. Thus the sets of models for analysing stream networks and corridors were not considered in this study. Firstly, AnnAGNPS was calibrated for runoff by modifying Curve Number (CN) values for different stages of the main crops. Results showed that the model satisfactory simulated surface runoff at annual, monthly and seasonal scales both in calibration and in validation processes. A Differential Sensitivity Analysis (DSA) was carried out to evaluate the sensitivity of seven input parameters for sediment load prediction. Based on DSA results, the AnnAGNPS model was calibrated for sediment load simulation. The model was capable of simulating the sediment load at an annual scale with a difference of less than 1% for calibration and 7% for validation. However, the results at monthly and seasonal scale were less precise. The model response can be considered as satisfactory for determining best management practices (BMPs), and for risk & cost/benefit analyses. The use of stream networks and corridors models, already inserted in the system of computer models that make up AnnAGNPS, and that include more detailed science for the channel hydraulics, morphology, and transport of sediments, seem to be necessary for a more precise explanation of sediment yields and loads at event, seasonal and monthly scales, even for small watersheds.