Hydrological Behavior Of A Semi-Dry Forest In Northern Mexico: Factors Controlling Surface Runoff

The understanding of the hydrological processes in transitional zones between semi-arid and temperate ecosystems is essential since these areas are particularly sensitive to changes in precipitation and land cover. The objectives were to (1) determine the most critical factors in the generation of surface runoff during one rainy season; and (2) analyze the effect of different rainfall variables and soil moisture on the amount of surface runoff. The study area is in a semi-dry forest in the state of Chihuahua, Mexico. Vegetation, soil, and precipitation variables were monitored on three micro-catchments (0.19-0.27 ha) in 2018. Maximum 30-min rainfall intensity, rainfall depth, and soil moisture were the main variables controlling surface runoff. Rainfall intensity was better related to the amount of surface runoff, with an increase in runoff when rainfall intensities were greater than 25 mm h(-1). The highest runoff coefficients (10%) occurred only when rainfall intensity exceeded 30 mm h(-1). Similarly, surface runoff was only activated when rainfall depth exceeded 20 mm, or when the maximum volumetric water content by rainfall event exceeded 30%. Only 10% of storms exceeded these thresholds, indicating the difficulty of runoff generation in transitional zones. The interception (36%; s.d. = 30.7%), and the drastic decrease in soil moisture after each storm, suggest that more than 90% of the rainfall was lost by evapotranspiration. Our findings suggest that the groundwater recharge in a given year in these transitional zones is highly related to the number of low intensity and long duration rainfall events.