Ecological Site Descriptions and Hydrologic Assessment

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1 Ecological Site Descriptions and Hydrologic Assessment Frederick B. Pierson Research Leader and Rangeland Hydrologist USDA Agricultural Research Service Northwest Watershed Research Center, Boise, Idaho

2 Hydrologically Stable Rangelands Interpreting Indicators of Rangeland Health, Pellant et al Stable rangelands effectively capture and store soil and water resources, safely release water, resist alteration of these functions, and recover these capacities postdisturbance. Hydrologic function is maintained by site specific structural and spatial vegetation and soil arrangements that elicit hydrologic processes favoring soil and water retention.

3 Runoff and Erosion Hydrologic Risk Hydrologic Response as a Function of Ecological State Rainfall High Low Hydrologically Stable Transition Ecological State Hydrologically Unstable A B SagebrushSteppe PinyonJuniper

4 Hydrologic Vulnerability Hydrologic Risk Hydrologic Stability Runoff and Erosion Overalnd Flow Velocity Storm Return Interval 100 yr 75 yr 50 yr 25 yr RainsplashSheetflow Hydrologically Stable Hydro Process Transition Zone Concentrated Flow Hydrologically Unstable Site Susceptibility Bare Ground Ground Cover Surface Roughness Soil Water Repellency Aggregate Stability

5 Drivers of Hydrologic Response Precipitation type and regime: Rain vs. snow Runoff (mm h 1 ) mm h 1 Runoff 64 mm h 1 Sediment 102 mm h 1 Runoff 102 mm h 1 Sediment Sediment Yield (g m2) :00 05:00 10:00 15:00 20:00 25:00 30:00 35:00 Time (mm:ss) 1. Frontal vs convective storms 2. Impacts of large rare events

6 Controls on Hydrologic Response (Site Susceptibility) Site/Soil Characteristics: Slope/topography Soil texture Bulk density Soil structure Soil organic matter Aggregate stability Soil moisture Soil water repellency Surface rock

7 Controls on Hydrologic Response Vegetation Cover: (Site Susceptibility) Canopy cover Rainfall interception and energy dissipation Ground cover (plant bases and litter) Surface water storage and resistance to overland flow Bulk density, organic carbon, soil macropores Degree of impact correlated with plant species/life form

8 Site Susceptibility (Risk) Shifts in Hydrologic Stability Site Degradation Decreased interception and surface storage Decreased infiltration Rapid runoff generation Reduced surface roughness and flow dissipation Initiation of concentrated flow with increased flow velocity Effects exponential with runoff volume rainfall intensity Ecological degradation Hydrological degradation

9 Reference State Degraded State Decreasing runoff and erosion with increasing area: splashsheet processes dominate. Increasing runoff and erosion with increasing area: concentrated flow processes dominate.

10 Juniper Encroachment Concentrated Flow Erosion SagebrushSteppe Woodland 40 % bare 1 Hydrologic Vulnerability Storm Return Interval 100 yr 75 yr 50 yr 25 yr RainsplashSheetflow Sagebrush Steppe Hydro Process Transition Zone Phase I II PJ 1 2 Concentrated Flow Phase III PJ Site Susceptibility 1) Sagebrush Steppe 20 g 2) Juniper Present Woodland 45 g 3) Juniper Dominated Woodland 145 g 3 60 % bare 2 85 % bare 3

11 Postfire Sediment Yield SagebrushSteppe 1 Hydrologic Vulnerability Storm Return Interval 100 yr 75 yr 50 yr 25 yr RainsplashSheetflow 2 Years Postfire 3 Hydro Process Transition Zone 1 Year Postfire 2 Site Susceptibility Concentrated Flow Immediately Postfire 1 76 % bare 64 % bare 2 3 1) Immediately postfire 988 g m 2 2) 1 year postfire 296 g m 2 3) 2 years postfire 6 g m 2 37 % bare

12 Predictive Understanding = Available Data * * * * * ** ** *** * * * * * * * * WEPP NRCS NRST ARS Scientific Studies Research over past decades has resulted in data sets from numerous ecological sites. Limits of current data: 1. Data mostly from gently sloping rangelands 2. Over small spatial scales 3. Static state conditions 4. Static rainfall intensities

13 Spatial Effects Vegetation Heterogeneous Cover/Soils Hydrologically Stable Vegetation Gaps = Indicator Hydrologic Function Sediment Yield (g m2) Small Plots Large Plots Coarsened Structure Hydrologically Unstable Sediment Yield (g m 2 ) Small Plots Large Plots

14 Temporal Variability Infiltration (mm h 1 ) Year 1 Unburned Year 2 Unburned Sagebrush Steppe, Breaks, ID, USA Unburned Time (min) Burned Year 1 Burned Year 2 Burned WDPT (s)

15 Ecological Site State Transitions ShrubSteppe ShrubAnnual Grass PinyonJuniper Annual Grassland Prioritizing efforts Where to focus resources to gain predictive understanding?

16 Hydrologic Vulnerability Hydrologic Risk ESDs and Hydrologic Assessment Runoff and Erosion Overalnd Flow Velocity Storm Return Interval 100 yr 75 yr 50 yr 25 yr RainsplashSheetflow Hydrologically Stable Hydro Process Transition Zone Concentrated Flow Hydrologically Unstable Establish relationships between controls of site susceptibility and site vulnerability Quantify variations in site susceptibility within reference state and changes in state Site Susceptibility Bare Ground Ground Cover Surface Roughness Soil Water Repellency Aggregate Stability Establish probabilistic riskbased decision making framework for land managers.