A methodology to improve the accuracy of Total phosphorous diffuse load estimates from agroforestry watersheds

This study was undertaken to improve the accuracy of Total Phosphorus export coefficient models; an essential need for the water management process. The modeling approach used in this study was supported by the application of four different models to 27 agroforestry watersheds located in the Mediterranean region. The first model definition relies on the correlation between the time of concentration and the Total Phosphorus export coefficient for each watershed. The second model predicts nutrient loads in the watersheds considering export coefficients reported in the literature and watershed land use areas. As an alternative approach, the remaining two models solve a system of equations through the application of a Bayesian optimization algorithm that iteratively modifies the land use export coefficients to minimize the error between the observed and predicted Total Phosphorus export coefficients. The difference between both models relies on the constraints imposed to the variation of the export coefficients. The modeling approach also included watershed clustering and the consideration of a correction factor, computed as absolute differences (observation-prediction). This correction factor encapsulates the widely accepted conclusion that the sediment delivery ratio of a watershed tends to decrease in line with an increase in drainage area, and that a significant amount of the Total Phosphorus transported in rivers is in particulate form and is primarily correlated to suspended solids. The results of the study clearly show that the consideration of Total Phosphorus export coefficients obtained from the literature for watersheds located in the Mediterranean region produce very poor Total Phosphorus diffuse load estimates. When considering the results of all the models, the Percentual bias and Nash-Sutcliffe model efficiency values varied from-80.2 % to 24.7 % (mu =-52.5 +/- 20.2) and-8.8 to-1.8 (mu =-4.3 +/- 2.8). However, the modeling approach considered in this study generated significant improvements in terms of the accuracy of the Total Phosphorus export coefficient from different land use classes and, consequently, the overall estimation of Total Phosphorus diffuse loads from agroforestry watersheds. When considering the results of all the models, the Percentual bias and Nash-Sutcliffe model efficiency values varied from-13.3 % to-5.0 % (mu =-9.0 +/- 2.9) and 0.5 to 0.9 (mu = 0.8 +/- 0.2), respectively. Overall, the results of this study suggest that the solution proposed has the potential to significantly improve the quantification of Total P diffuse loads at the watershed level, which is an important conclusion given the present and future challenges currently being faced with regard to the water management process.