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Evaluating the Effects of Grazing Land Conservation Practices on Southwestern Watersheds

McClaran, Mitch
University of Arizona
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The long-term goal of the project is to develop tools to estimate the combined effect of grazing land conservation practices and drought on watershed health and to develop tools for watershed health assessment.

The first objective is to assemble and analyze the time series of plot-based measures of vegetation recovery from drought for single and combined conservation practices. Work in the first objective will address the following hypotheses: Recovery following drought does not differ between seasonal rotation and yearlong grazing practices, among combinations of grazing management and fire practices, and among grazing management and brush control practices.

The second objective is to improve the capabilities of the R.AGWA erosion-sediment yield model to compute sediment budgets for a watershed and provide information on the effects of grazing land conservation practices on those sediment budgets.

The third objective is to use simulations of parameterized watershed-scale models to estimate optimal combinations and spatial arrangements of conservation practices to maintain and improve watershed health. Work in the third objective will address the hypothesis that the combination and spatial arrangement of grazing lands conservation practices will affect response (magnitude and rate of recovery) to drought.

The fourth objective is to describe the current socio-economic drivers influencing the adoption and maintenance of conservation practices in response to drought, and evaluate the utility of simulation models to influence future adoption and maintenance of conservation practices. Work in this fourth objective will address the hypothesis that drought modifies livestock growers decisions, potentially leading to negative consequences.

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Non-Technical Summary: Conservation of soil and water is the keystone to sustainable livestock grazing and maintenance of native species on Southwestern grazing lands. The negative impacts of erosion on vegetation productivity can have significant economic impacts to a ranch operation and sedimentation is the leading water quality problem in the western United States impacting reservoirs and aquatic environments. Recurring drought conditions can override the success of conservation practices in the arid and semiarid Southwest. The severity and persistence of these drought-related impacts to watershed health can vary among conservation practices. Therefore, government assistance programs intended to support soil and water conservation practices need to assess their effectiveness before, during and after drought. We focus our project on identifying optimal suites of conservation practices and their spatial arrangement to minimize the drought-associated declines in watershed health, and to support faster recovery following drought. This attention to rates of recovery or resilience, is consistent with the concept of self-repair; where conservation practices would facilitate faster self-repair and support long-term health of watersheds. In addition, our attention to drought-related responses will contribute to drought management guidelines that will become increasingly more critical if predictions of drier climates occur in the future. The long-term goal of the project is to develop tools to estimate the combined effect of grazing land conservation practices and drought on watershed health and develop tools for watershed health assessment. To reach this goal, our proposed project will analyze long-term, vegetation and erosion data from the Santa Rita Experimental Range, Arizona and Walnut Gulch Experimental Watershed, Arizona to understand the simple and combined effects of prescribed grazing, fire, and brush control on the response to and recovery from drought. The vegetation analysis results will be used to develop a model simulating soil erosion and water runoff. That model will be used to simulate erosion for a watershed that would be affected by the combination conservation practices and recurrence of drought conditions. Through outreach and extension, we will evaluate the effectiveness of these simulations in modifying stakeholder behavior regarding the adoption and maintenance of conservation practices that protect and improve watershed health.

Approach: To address Objective 1, we will use long-term plot-scale vegetation measurements collected on the Santa Rita Experimental Range (SRER), Arizona for three conservation practice cases: seasonal rotation versus yearlong grazing, a factorial combination of high intensity-low frequency versus yearlong grazing combined with brush control fire or no-fire, and a factorial combination of seasonal rotation versus yearlong grazing combined with chemical brush removal or no removal. Each case spans the most recent drought (2000-2006), and cases 1 and 3 span the 1978-80 drought. The return to average and above average precipitation beginning in summer 2006 provides the post-drought conditions necessary for this analysis. We will measure cover and density in 2009 to provide the most recent vegetation response to conservation practices following the recent drought. We will compare the vegetation resilience following drought as the dependent variable. Recovery will be calculated as the proportion of pre-drought vegetation present at a given time following the cessation of the drought. The different management practice combinations will serve as the independent variables. To address Objective 2. we will develop a simple model based on results from Objective 1 to simulate changes in vegetation as the function of climate and conservation practices. We will develop a surface-roughness parameterization procedure by calibrating KINEROS2 under different vegetation conditions and conservation practices using empirical data relative to land use treatments from instrumented watershed at SRER and Walnut Gulch Experimental Watershed, Arizona. We will modify the KINEROS2 model to improve channel routing and sediment transport using diffusion-wave modeling. To address Objective 3, we will use results from Objectives 1 and 2, to simulate responses to a suite of cases using the modified version of R.AGWA. Simulations will run for at least 25 years and each case will be simulated at least 50 times with different climate files using a Monte Carlo process. The endpoints will be hillslope erosion (tonnes/ha), runoff (mm), peak discharge (mm/sec) and sediment yield (tonnes/ha). The stochastic weather generator, CLIGEN will be used to create the climate files. At least one drought cycle and a severe drought series will be included each climate time series. To address the fourth objective, we will use responses to two questionnaires completed by stakeholders involved in range extension programs at the University of Arizona. The first questionnaire will describe the conservation practices currently being used, rational for their use, and why practices may be altered in response to drought and changes in financial support. These responses will be used to create the case studies used in Objective 3. After the results of the simulations are presented to stakeholders, the second questionnaire will be used to describe how attitudes change toward their conservation practices have changed. It will also address potential barriers to adopting new management practices identified in the simulations, and information needed to assist stakeholders to implement conservation practices.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Sanitation and Quality Standards
Prevention and Control
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