Submitted to: Hydrological Processes
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/4/2006
Publication Date: 4/20/2006
Citation: Seyfried, M.S., Wilcox, B. 2006. Soil Water Storage and Rooting Depth: Key Factors Controlling Recharge on Rangelands. Hydrological Processes, 20:3261-3275. Interpretive Summary: In the past one hundred years there has been a dramatic increase in woody vegetation on rangelands of the western USA. Currently, consider interest in returning those lands to pre-settlement conditions, partly to increase forage, but also to possibly increase water supply. Although it is widely believed that removal of woody vegetation will result in increased water supply, data addressing the topic are mixed. We proposed an approach to analyzing the likelihood of woody vegetation actually increasing water supply and then demonstrated its application using data collected from sagebrush-bitterbrush rangeland. We found that there was an increase in water supply at the site of about 6 cm of water per year. This rather large gain will apply only to a small part of the region, however, because it is only expressed where soils are relatively deep (deeper than about 4 feet). From a scientific perspective, the paper demonstrates a conceptual approach the water balance changes with vegetation. The results also have important management implications because they show that the success vegetation management program is highly site dependent. We provide parameters and procedures for estimating the likelihood of increasing water supply following removal of woody vegetation.
Technical Abstract: The practice of removing woody vegetation to enhance water supply in semiarid rangelands in the USA continues to generate considerable interest, even though past research has yielded apparently contradictory results concerning its efficacy. In an attempt to elucidate the factors that determine whether and how woody vegetation removal affects water supply, we analyzed the problem using a water balance approach. In our analysis, deep drainage is the water balance component associated with water supply. Because the herbaceous vegetation that replaces the woody plants generally has a shallower effective rooting depth (Rd), the amount of soil water potentially available for transpiration is reduced and more is available for deep drainage. The potential increase in deep drainage can be estimated from the capacity of the soil to store plant-available water (Sc) and may be substantial. Our case study on sagebrush rangeland documents how Rd, and consequently Sc, changed after woody vegetation at the site was removed by burning. Using depth profiles of soil water content and matric potential, we showed that the Rd of the post-fire vegetation was about 140 cm, 60 cm less than that of the pre-fire vegetation, and that this resulted in a potential increase in deep drainage of about 6 cm of water—which in semiarid rangelands is substantial. Historical precipitation patterns indicate that there is nearly always sufficient net precipitation to generate the additional 6 cm of deep drainage at this site. However, in most of the area the soil depth is less than 140 cm, so that transpiration and deep drainage would be unaffected by the vegetation change in most of the area and the overall water supply enhancement would be much less than 6 cm. These results indicate that the change in Sc that may follow woody shrub removal is an important criterion to evaluate sites for vegetation conversion.