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Hydrological Impacts of Woody Plant Encroachment in Arid and Semiarid GrasslandsEPA Grant Number: F6F11227
Title: Hydrological Impacts of Woody Plant Encroachment in Arid and Semiarid Grasslands
Investigators: Kim, John H.
Institution: Duke University
EPA Project Officer: Manty, Dale
Project Period: September 1, 2006 through September 1, 2008
Project Amount: $111,172
RFA: STAR Graduate Fellowships (2006) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Ecological Indicators/Assessment/Restoration , Fellowship - Ecohydrology
Understanding how these changes modify hydrological processes is vital for effective management of water and vegetation. This research will focus on the ecosystem level water-use changes brought on by vegetation changes and human land-use. How these changes affect the various components of the hydrological cycle will be assessed.
I will assess the effects of vegetation changes (woody shrubs replacing grassland) and land-use on the hydrological cycle and focus on groundwater recharge, thereby developing a better framework for ecohydrology in arid regions. Combining rainfall simulations in paired grassland/shrub sites across different management practices, I hope to determine the effect of woody shrubs on runoff and recharge. I will explore the following questions: Question 1: How are the hydrological processes affected by woody plant encroachment? Question 2: How do different land management practices, namely discing (a method of shrub control in many areas) and grazing affect hydraulic properties of soil? How do the spatial scale of observation and different rainfall intensities affect the hydrological impact of land-use?
Rainfall will be applied above the canopy level on paired shrub/native grassland sites as well as in different land-use areas (grassland, grazed, disced). The plots will be monitored continuously for water budget including precipitation, soil water, transpiration, interception, runoff, subsurface flow, recharge, and vegetation and soil characteristics. A set of rainfall simulation experiments will consist of 1) a short, intense rainfall event with an application rate of 100 mm/hour for one hour, and 2) 10 mm/hour for 24 hours. I will use the two intensities of rainfall simulation to determine the effect of rainfall intensity on runoff. These combined efforts will give nested data on runoff at the microplot (0.5 m2), plot (36 m2), and small-catchment scales (Honey Creek Watershed measurements). In addition, these data may be compared with those from the large-scale runoff monitoring being carried out by the USGS in the adjacent Guadalupe River basin. Runoff data collected from this study and ongoing data-collection by the NRCS and the USGS will form the basis for comparisons of runoff at multiple scales. Data loggers will be used to collect data from sapflux sensors, rain gauges with float and potentiometer, tipping bucket gauges, flume with electronic flow meters, and ponded/tension infiltrometers. Vegetation cover will be assessed with recent aerial photographs.
Woody plants may be able to access deeper groundwater for year-round transpiration and thus consume more water than grasses, affecting recharge, soil moisture and runoff. Amount of water available to plants from precipitation is determined in part by nfiltration rates into the soil. Intense grazing can increase soil compaction and hence runoff. Current management efforts to curtail the encroachment such as discing may enhance infiltration rates, although loosened soil can be lost as runoff in intense precipitation events. These processes can affected by rainfall intensity and can be independent of scale.