This research project evaluated the effect of two biophysical factors, weather and land cover, on the water availability estimated for the Conservation Area of Guanacaste (ACG for its acronym in Spanish) located in the northwestern part of Costa Rica. Water availability has been calculated using an advance and well-studied hydrological model, the Soil and Water Assessment Tool (SWAT), which simulates all the processes of the hydrologic cycle based on the water balance equation. From SWAT estimations, water yield was selected as the variable that quantifies the water available in different geographic units; in this case, 107 sub-basins identified within the drainage area (2,861.9 km²) of the ACG. These sub-basins were grouped into four major basins identified in the official cartographic information of Costa Rica, namely: Nicaragua Lake, Santa Elena Bay, Papagayo Gulf, and Tempisque River, which comprise 39.9%, 7.3%, 5.0%, and 47.7% of the total drainage area, respectively.

Regarding the biophysical factors, the weather parameters of mean annual precipitation (MAP) and temperature (MAT) data were obtained from the Climate-South America (ClimateSA) software. For the second biophysical factor, land cover maps from 1979, 1997, and 2015 were examined to determine the area [km²] in each sub-basin of four classes: 1. Agricultural land (AGRL), 2. Deciduous Forest (FRSD), 3. Evergreen Forest (FRSE), and 4. Pastures/Hay (PAST).

I observed that the ACG's water yield tends to rise over time, approximately 138.5 mm (p-value: 5.8 X 10-67) per decade. Of the two biophysical factors studied, climate is the one that could be linearly related to the increasing trend in water availability. This factor changed 11. 7 mm of rainfall (p-value: 0.03) and 0.1 °C (p-value: 8.4 X 10-13) every ten years.  

This study offered essential baseline projections in water availability and climate changes for decision-making to identify anomalies that generate warnings for preserving all ecosystem services that depend on water availability in the ACG, which is a World Heritage Site with a high ecological value for humanity. In addition, this project also opened the discussion to investigate the effect of land cover further, as several studies in Central America have shown a positive impact of forest restoration on water yield. Also, to assess other biophysical factors, such as topography, which determines the spatial stratification of hydrological and climatic variables. 

Finally, the achievement of this project demonstrates the importance of conducting thorough statistical analyses with knowledge of the population under study, such as the inclusion of covariates to minimize confounding factors, the definition of the type of variables (categorical vs. continuous numerical), and the elimination of outliers that may be due to measurement errors.