Evaluation of an antecedent precipitation index to model runoff yield in the western Sierra Madre (North-west Mexico)

In a subtropical mountain of northern Mexico, soils and slopes seem to have a hortonian functioning, as it is commonly observed in other tropical or subtropical areas. So it is important to know previous soil moisture before any rainfall event in order to properly estimate the runoff coefficients and to get a better understanding of the hydrologic behaviour of the catchments. A very simple deterministic model, named NAZASM, is developed and tested for 6 microplots (1 and 10 m2), 13 plots (50 m2) and 5 catchments (1–50 km2). It is based on the ‘antecedent precipitation index’ with an exponential decay parameter (α) into the rainfall–runoff relationship. Measurements of rainfall and runoff were made during four rainy seasons. Between 22 and 157 rain events were considered in the analysis, depending on the sizes of the plots and catchments. ‘NAZASM’ gives reasonable estimations of runoff if the calibration data set includes events occurring during wet and dry years, due to the difference of hydrological processes involved in these two situations. All the results have shown that the use of such a model significantly improves the explained variances between measured runoff and rainfall as compared to classical regression analysis, whatever the size of plots and catchments are. The α soil water content decay parameter which in theory is a soil characteristic, appeared in fact to be scale dependent.