Long‐term trends in streamflow from semiarid rangelands: uncovering drivers of change

Abstract

In the last 100 years or so, desertification, degradation, and woody plant encroachment have altered huge tracts of semiarid rangelands. It is expected that the changes thus brought about significantly affect water balance in these regions; and in fact, at the headwater-catchment and smaller scales, such effects are reasonably well documented. For larger scales, however, there is surprisingly little documentation of hydrological change. In this paper, we evaluate the extent to which streamflow from large rangeland watersheds in central Texas has changed concurrent with the dramatic shifts in vegetation cover (transition from pristine prairie to degraded grassland to woodland/savanna) that have taken place during the last century. Our study focused on the three watersheds that supply the major tributaries of the Concho River -those of the North Concho (3279 km 2 ), the Middle Concho (5398 km 2 ), and the South Concho (1070 km 2 ). Using data from the period of record , we found that annual streamflow for the North Concho decreased by about 70% between 1960 and 2005. Not only did we find no downtrend in precipitation that might explain this reduced flow, we found no corresponding change in annual streamflow for the other two watersheds (which have more karst parent material). When we analyzed trends in baseflow (contributions from groundwater) and stormflow (runoff events linked to specific precipitation events), however, we found that in spite of large increases in woody plants, baseflow for all the watersheds has remained essentially consistent or has increased slightly since 1960. At the same time, stormflows were of smaller magnitude. Animal numbers have declined precipitously in the latter half of the last century. We suggest that these lower stormflows result from generally higher soil infiltrability due to generally improving range condition. There is no indication that the decline in streamflow is related to diminished groundwater flows caused by extraction of subsurface water by woody plants.

Biohydrologic effects of eastern redcedar encroachment into grassland, Oklahoma, USA

“…However, whether or not any increases in evapotranspiration are discernable at the watershed scale will also depend on the proportion of the watershed affected by encroachment. The present study concurs with past work in suggesting that while woody encroachment can increase evapotranspiration significantly, those increases are strongly dependent on the degree to which the site was degraded prior to woody encroachment (Wilcox et al 2008).…”

Streamflow Regime Variations Following Ecological Management on the Loess Plateau, China

“…The trends were determined by applying the BS-MK and Sen tests to the annual and daily series of streamflow series. Incremental percentiles on an annual basis were used to obtain data series [44]. The observed data from each year were sorted in descending order.…”

Annual baseflow variations as influenced by climate variability and agricultural land use change in the Missouri River Basin

“…In the Southern Great Plains, researchers reported increasing trends in baseflow with tree cover increased (Wine and Zou, 2012). Wilcox et al (2008b) also showed that increased baseflow, in central Texas watersheds, was attributable to deeply rooted trees and greater infiltration capacity of soils as degraded grasslands were converted to woodlands. A strip of forest, grassland, and shrubland is also observable in the Ozark Plateaus in Missouri, downstream of the basin, leading to relatively elevated baseflow yield ( Fig.…”