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by: Daniel DietzWith much of Texas suffering an extreme drought - October to April was the driest seven-month period in recorded history - it is not surprising that many of us have water on our mind. One question that often arises in central and western portions of the state is whether or not managing the surface vegetation can increase the amount of water available for our aquifers and reservoirs, and, if so, by how much.
The short answer is that managing some types of woody vegetation can increase the water yield from a property, but by how much depends on the type of rainfall events that are experienced and the underlying geology. The good news is that when tree and brush control is undertaken in the appropriate location, it can not only increase the amount of water available, but it can help maintain our native grasslands.
First, let’s review the water cycle (left). Most precipitation does not enter aquifers or creeks, but is returned to the atmosphere through evaporation (from either plant surfaces or soil) or transpiration (water released by plants during photosynthesis and respiration). These two processes are collectively referred to as evapotranspiration. Not all plants have the same rate of evapotranspiration. Studies suggest that woody plant species evapotranspire more water than grass species. They intercept more rainfall in their canopies since they have a greater leaf surface area than grasses, which is then lost to evaporation. When trees and brush are removed and replaced with native grasses, more of the water reaches the soil. If these studies are correct, the replacement of trees and brush species with grasses can lead to greater water infiltration and runoff.
While trees and brush may evapotranspire at higher rates than herbaceous vegetation, not all tree species are the same. Surprisingly, mesquite removal does not have a big impact on water yield according to a recent Texas Tech literature review, but juniper removal can have pronounced impacts. Junipers, also commonly called cedar, have greater average evapotranspiration rates than live oaks or mesquite. A Juniper can transpire almost twice as much water as an oak with the same canopy volume. Junipers have three times more leaf surface area than live oaks per unit of canopy. Juniper often transpires during periods of water stress when live oak decreases photosynthesis and transpiration rates.
With its dense growth and high leaf surface area, Juniper also intercepts more rainfall in its leaves and prevents the water from ever reaching the soil. How much depends on the type of rainfall events that are occurring, but a recent study in the southern Edwards plateau found that the Juniper canopy intercepted 40% of the rainfall over a 3 year period.
The ratio of grass cover to tree cover which provides the most benefit to water quantity is not entirely known. One study indicates that significant water yield gains can be had by reducing tree and brush cover below 30%, whereas another study states that significant gains in water yield may not be seen unless tree and shrub cover is reduced below 15%. How much water can be gained in this way? Once again there is not consensus on this question, but a study by Tom Thurow estimated that moving from 75% brush cover to 15% brush cover would yield an additional 20,000 gallons of water infiltration per acre per year in the western Edwards plateau where rainfall averages 27.5 inches a year.
Increasing water yield and maintaining our native grasslands are important. However, before we all sharpen our chainsaws and fill up our drip torches, let’s keep in mind that there are many areas where brush control may not be appropriate. Trees and shrubs are important components of riparian areas and perform many important functions there. Extensive land clearing on slopes may cause soil erosion if not revegetated properly. Some research has suggested that dense juniper stands may aid soil formation in areas that have experienced erosion. And let’s not forget that the only songbird that nests exclusively in central Texas, the federally endangered golden-cheeked warbler, can only be found in mature Juniper-oak woodlands.
Graphic: The most common effects of vegetation on precipitation in a rangeland system. Vegetation returns precipitation to the atmosphere by intercepting rainfall before it reaches the soil or by removing it from the soil and releasing it as water vapor as part of photosynthesis and respiration. The vast majority of the remainder either moves overland to bodies of surface water or percolates to the water table. Illustration by Matt McCaw.