Bradford and others Ecohydrology in adjacent sagebrush lodgepole-pine ecosystems

Appendix 3. Validation of Sagebrush Ecosystems Representation in SOILWAT

SOILWAT, a daily time step soil water simulation model, was developed and tested in the semiarid western US shortgrass steppe (Parton 1978; Sala and others 1992) and previously used in shrublands (Bradford and Lauenroth 2006; Schlaepfer and others 2012; Schlaepfer and others In Press). We compared SOILWAT model output with measured field data from the literature that met the following criteria: water balance measurement data is available and accompanied with a detailed soil and site description. Unfortunately, only a study with a sagebrush-grass site from southwestern Idaho, Reynolds Creek Experimental Watershed (Wight and others 1986) met enough criteria for a comparison. The site is a good representation of the median of sagebrush ecosystems, for example, soils are loam and MAP is 349 mm (compare Table 1). We ran SOILWAT using climate and soils inputs appropriate for Reynolds Creek, as detailed in (Schlaepfer and others In Press). This procedure omitted information on local conditions (for example, weather station and soil profiles) and potentially causes worse than possible predictions; meanwhile, it guarantees comparability with the simulation results.

SOILWAT estimatesof average daily volumetric soil water content are comparable with measured values (Figure A3.1). SOILWAT estimates of actual evapotranspiration (AET) described the pattern of inter- and intra-annual dynamics (FigureA3.2). The different measurements and simulations of AET reported by Wight and others(1986) show a variability both in timing and size of mid-summer peak of AET. SOILWAT predicted values for mid-summer peak of AET in the range of the published spread. A second late-season peak in AET is well captured by SOILWAT. The dynamics of transpiration were also well described by SOILWAT (FigureA3.3). Both in 1978 and 1979, published transpiration values were comparable to SOILWAT simulations, but in 1977, SOILWAT simulated smaller transpiration values. The discrepancy in the year 1977 could be explained by precipitation inputs being derived from the gridded dataset compared to on-site measurements by Wight and others(1986). We conclude that the proposed representation of sagebrush ecosystems in SOILWAT using the described data sources is realistic.

FigureA3.1. Comparison between measured (black) and SOILWAT modeled (red) average daily volumetric soil water content (VWC) over12 years (1999 to 2011) for the Reynolds Creek Experimental Watershed in southwestern Idaho. VWC was measured and modeled at 5 depths (bottom 5 panels). Shaded areas around the lines are 1SD.

Also included for comparison are mean daily maximum and minimum temperature (top panel), precipitation (blue; second panel), infiltration (light blue; second panel), snow water equivalent (measured and modeled in blue and purple, respectively; third panel), and number of years of observations for each day of year (gray; fourth panel).

FigureA3.2. Comparison between evapotranspiration (ET) in the years 1977 to 1979 simulated by SOILWAT for the Reynolds Creek Experimental Watershed in southwestern Idaho and ET data measured (Lysimeter, Water Balance), simulated (SPAW, CREAMS, ERHYM) and averaged (Wight and others 1986).

FigureA3.3. Comparison between transpiration in the years 1977 to 1979 simulated by SOILWAT for the Reynolds Creek Experimental Watershed in southwestern Idaho and data simulated (SPAW, CREAMS, ERHYM) from Wight and others(1986).

References

Bradford, J.B. & Lauenroth, W.K. (2006) Controls over invasion of Bromus tectorum: The importance of climate, soil, disturbance and seed availability. Journal of Vegetation Science,17, 693-704.

Parton, W.J. (1978) Abiotic section of ELM. Grassland simulation model (ed. G.S. Innis), pp. 31-53.Springer-Verlag Inc., New York.

Sala, O.E., Lauenroth, W.K. & Parton, W.J. (1992) Long-term soil-water dynamics in the shortgrass steppe. Ecology,73, 1175-1181.

Schlaepfer, D.R., Lauenroth, W.K. & Bradford, J.B. (2012) Effects of ecohydrological variables on current and future ranges, local suitability patterns, and model accuracy in big sagebrush. Ecography,35, 374-384.

Schlaepfer, D.R., Lauenroth, W.K. & Bradford, J.B. (In Press) Ecohydrological niche of sagebrush ecosystems. Ecohydrology.

Wight, J.R., Hanson, C.L. & Cooley, K.R. (1986) Modeling evapotranspiration from sagebrush-grass rangeland. Journal of Range Management,39, 81-85.

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