Llano Estacado: Landscape design alternatives for precipitation management
Watson, Tyson Eli
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Lubbock County, Texas, in the southern portion of the High Plains ecoregion called the Llano Estacado, was historically characterized by short-grass prairie and bison herds. Today this area has a growing and urbanizing population, rapidly declining groundwater supply, and is semi-arid receiving only 18 inches of precipitation annually (Texas Parks and Wildlife 2017). The physiography is generally considered flat with an expansive horizon. A more detailed investigation reveals tens of thousands of shallow bowl-like depressions dimpling the surface with ephemeral playa lakes defined by Randall clay pans in their low points. Very few streams and rivers cross this vast plain, so surface water is scarce over large land areas during seasonal dry periods and droughts. The region has quality agricultural soils and sits atop the High Plains/Ogallala Aquifer, as well as the Permian shale oil and gas formation. The region is dependent upon the finite groundwater resources of the High Plains/Ogallala Aquifer for human consumption, and row crop agricultural production that is the base of the regional economy. While the playa’s ephemeral qualities do not offer continuous surface water resources for consumption, or agricultural use, the clay pans dry and crack becoming the primary source of groundwater recharge for the High Plains/Ogallala Aquifer. Temporal wet periods in the playas create biodiversity hot spots along their edges in the annulus zone, and serve as the ecosystem forage and habitat host to migratory waterfowl and invertebrate species. Playa lakes are critically important, however are not preserved, conserved, or managed to maintain their important hydrologic and ecologic functions or services in current agricultural or suburban development. The City of Lubbock is the largest urban center in the region with an MSA (Metropolitan Statistical Area) population of 307,714 (2015) project to add 21,547 people by 2020, and grow to 379,545 by 2030 (Texas Demographic Center 2018). Given the rapid population growth, land use is changing from agriculture production to suburban single family detached developments with ranch style homes with large roof areas. While groundwater used for crop irrigation decreases in this development scenario, suburban water use and stormwater runoff increases dramatically. Texas Water Development Board (TWDB) statistics indicate by 2020 both the irrigation and urban demand for Lubbock County will exceed the existing supply. The High Plains Water District (HPWD) 2018 water level measurements indicated an average decline in aquifer levels of -0.16 feet and average saturated thickness at a very thin 56 feet. This case study employs a combination of landscape architecture and geodesign methods to understand how agricultural crop implementation and typical single family developments can be redesigned to increase soil moisture via infiltration to decrease crop and landscape irrigation water use, protect playa flood plain areas, and maintain pre-development playa hydrology to maximize potential recharge, and regional biodiversity. Results indicate that pre-development playa runoff conditions can be met by proposed design alternatives implementing green infrastructure approaches.