Benefits of Forests for Water: Projected Effects of Land Use Change in the San Jacinto Watershed, Texas

ABSTRACT Land use change poses challenges to water supply sustainability. The San Jacinto watershed located in Southeastern Texas is a microcosm of these challenges, with high dependence on surface water supply and projected urban and agricultural expansion by 2070, with a corresponding decline in forest and natural grassland. This study used the Soil and Water Assessment tool (SWAT) to evaluate the effects of these land use changes on water quality and quantity in the San Jacinto watershed. Our results indicated that projected land use change could increase annual average river discharge by as much as 6%, sediment load by ~40%, total nitrogen load by ~10% and total phosphorous load by ~20% across major tributaries within the watershed. We also show that current land use in 2020 may have increased the annual average sediment loads by 13%–80% and nutrient loads by 14%–40% relative to natural land uses (e.g., forests and grasslands), underscoring the sustainable clean water supply of natural land use types. Furthermore, while our model showed land use changes can cause high local variations in hydrological impacts, the overall effect at the watershed scale was lower, emphasizing the importance of focusing on sensitive local areas. This study provides a comprehensive and high‐resolution assessment of the potential impacts of land use change on both water quantity and quality that can inform resource management decisions in a rapidly changing watershed.