Hydrologic effects and implications of vegetation in semiarid mountain regions Huade Guan Advisor: Dr. John Wilson SAHRA 4 th Annual meeting Oct. 15, 2004

South Baldy, Magdalena Mountains, New Mexico, 2001 N

SAHRA vegetation question: What are the impacts of vegetation change on the basin- scale water balance? Today I’ll focus on two issues related to understanding and modeling vegetation hydrologic effects in mountain areas. –Effects of vegetation on hydrologic processes –Mountain Block Recharge (MBR) at hillslope scale under various conditions, including vegetation types, and vegetation change SAHRA Scientific Question and My Study

Precipitation Bedrock Soil Soil water How does water partition on the mountain hillslopes? In particular, what is the percolation across the soil-bedrock interface? FS DS FR DR MASTER FAULT FAULT OBLIQUE FAULT Surface Fault Trace FAULT Evapotranspiration ? Precipitation Bedrock Interflow Percolation What is the contribution of distributed recharge to mountain-front recharge? From this percolation, what is the distributed recharge to the mountain block?

Effects of vegetation on hydrologic processes Modifies surface albedo Intercepts precipitation Transpires soil water, actively responds to the atmospheric condition and soil moisture Modifies soil structure and hydraulic conductivity Precipitation Bedrock Soil Soil water

Effects of vegetation on hydrologic processes We separate these effects into two categories, –one contributes to the boundary of the model: PE and PT (including Fr, albedo, interception, stomatal resistance, vegetation structure, etc); –the other contributes to the model parameters: K, root water uptake model.

Hydrologic effects of vegetation New surface energy partitioning model that we use to separately generate PE and PT on mountain hillslope: SEP4HillET Hydrologic modeling of the surface and vadose zone: currently HYDRUS PTPE Root macropore Modified from Shuttleworth and Wallace (1985)

SEP4HillET model: Considers effects of the vegetation coverage and slope (aspect and steepness) on surface energy partitioning for ET (E and T separately) modeling PE=83%, PT=17% PE=60%, PT=40% The model was tested for estimating both potential evaporation (PE) and potential transpiration (PT) on two surfaces of Sevilleta LTER by comparing to stable isotopic measurements (Boulanger,2003) Measured: E =79~84%, T = 16~21% Modeled: PE = 83%, PT = 17% Measured: E = 52~70%, T = 30~48% Modeled: PE = 60%, PT = 40% Modeled PE and PT shrub grass

N Test the hypothesis for distinct vegetation Different atmospheric demands for ET lead to different soil moisture regimes, and support different vegetation.

100 cm soil 30 cm soil Duration of dry root zone soil

N Creosote: P/PET ~ 0.18 Boundary P/PET ~ 0.2 Picture from Bruce Harrison Juniper: P/PET ~ 0.24 Model results with 8-year micrometeological Data at Red Tank station, Sevilleta LTER Long-term vegetation changes with climate

Mountain Block Recharge (MBR) at hillslope scale –under various conditions, including vegetation types, and vegetation change Precipitation Bedrock Soil Soil water

Granite Tuff Granite Tuff Annual P=565mm Fr=50% SN SN Annual P=565mm Fr=5% Percolation: in % of Precip Aspect effect 4% 31% 17% Aspect effect Vegetation control Soil and bedrock effects Generic hillslope hydrologic modeling MBR sensitivity to: bedrock permeability, soil thickness, vegetation coverage, and slope aspect. (climate variability, rainfall intensity, soil structure change and erosion due to vegetation change not considered) Soil 6% 7% 43% 22% 3% 1% 23% 6% 2% 0.3% 16% 1.8%

Los Alamos hillslope experiments (data from Newman, 2003) Simulations of Los Alamos hillslope experiments Site description: ponderosa pine, 6% slope, ~ 500 mm annual precip, permeable tuff However, little recharge Tuff

Simulations of Los Alamos hillslope experiments Lab measured K Soil is layered, with lowest measured hydraulic conductivity at 40 cm. However the soil moisture ponds at cm. Why? Tuff Root zone Impeding layer for percolation Because of root macropore !

Simulations of Los Alamos hillslope experiments root zone tuff barrier P=52cm root zone tuff root zone tuff P=52cm Ponderosa root zone tuff P=38cm Juniper Q1: Does ponderosa pine site naturally leads to soil impeding layer? Q2: Percolation < ?

Los Alamos hillslope experiments (data from Newman, 2003) Root zone Permeable Tuff Impeding layer for percolation Valles Caldera field sites Hillslope modeling What controls soil thickness at these sites? Impeding soil layer? What is the distributed MBR? Of the Los Alamos experiments suggests an impeding soil layer below the root zone of ponderosa pine forest. What about Valles Caldera?

Thank you ！