Groundwater in fractured granite: implications for tropical dry forest development and water sustainability

Abstract. Seasonality is one of the most important features of Tropical Dry Forests (TDFs), then water scarcity must be overcome by perennial sources dependent of groundwater flows. Groundwater recharge processes in TDFs are controlled by (i) the seasonal dynamics of the components of the water cycle through their interaction with the soil and underlying geological environment, and (ii) the phenological rhythms of the vegetation that simultaneously influence infiltration and evapotranspiration. The daily hydrological dynamics of a TDF that grow atop fractured granite with a thin layer of sandy soil were studied in three basins subject to conservation in the Pacific coast of southern Jalisco, Mexico along the 2019–2020 hydrological year. Automated climatological and streamflow instrumentation was used to obtain data for a detailed analysis of the rain-streamflow response and new instruments were placed to measure interception and soil moisture. Results show that annual precipitation was 1.179 mm (above the average of 832 mm) distributed in 80 highly variable events. The phenological stage of the vegetation and the accumulation of litter strongly influenced interception. Thin sandy soils (~ 0.30 m) controlled the rapid infiltration of 85 to 98 % of the precipitation that reached the ground along seasons, reducing the effect of evapotranspiration by percolation, aided by the fact that most of the precipitation events were nocturnal. The rain-streamflow response showed that groundwater discharge in the streams represented up to 70 % of the percolation volume and the remaining 30 % correspond to groundwater flow and temporary storage in the fractured medium. These two processes may explain the zonation of two subtypes of vegetation, their phenology and survival in the dry months. The deciduous tropical forest (DTF) in the study area developed in groundwater recharge zones, while the sub-deciduous tropical forest (SDTF) emerged in the discharge zones, where evapotranspiration values of up to 0.140 mm d^-1 were obtained from the diurnal variations of the base flow. Analyses of daily data highlight the importance of the fractured medium and its temporary saturation, where residence times may made water available in the ecosystem during dry periods. Improving our understanding of these processes will help guide the sustainability of provision of groundwater by the conservation of hydrological ecosystem services in the basins for anthropogenic activities in the region reducing hydrological vulnerability to dry periods and climate change.