This is a comparison of two very different hydrology models, both designed to predict runoff from ungaged rural catchments. One is the commonly used and conceptually simple Soil Conservation Service curve number method. The other is a process oriented model based on the Green and Ampt equation. The Green and Ampt model employs newly developed techniques for parameterizing the Green and Ampt equation based on readily available soil and vegetation information. Annual, monthly and daily predicted runoff were compared to observed on six uncalibrated rangeland catchments located in Texas, Oklahoma, Arizona, Nebraska and Idaho. Model parameterization was based strictly on individual catchment characteristics. No model calibration was performed. Results indicate that the Green and Ampt model is a potentially useful tool for predicting runoff. These results are important because they demonstrate the utility of complex physically based models as management tools for predicting land use impacts to runoff and infiltration.
Transport of mass and energy between and within soils, canopies, and the atmosphere is an area of increasing interest in hydrology and meteorology. On arid and semiarid rangelands, evapotranspiration (ET) can account for over 90% of the precipitation, making accurate knowledge of the surface energy balance particularly critical. Recent advances in measurement and modeling have made the accurate estimate of ET and the entire surface energy balance possible. The Simultaneous Heat and Water (SHAW) model, a detailed physical process model capable of simulating the effects of a multispecies plant canopy on heat and water transfer, was applied to 2 years of data collected for three vegetation types (low sagebrush, mountain big sagebrush, and aspen) on a semiarid watershed. Timing and magnitude of ET from the three sites differed considerably. Measured and simulated ET for approximately 26 days of measurement in 1990 were 41 and 44 mm, respectively, for the low sagebrush, 74 and 69 mm for the mountain big sagebrush, and 85 and 89 mm for the aspen. Simulated and measured cumulative ET for up to 85 days of measurement at the three sites in 1993 differed by 3-5%. Simulated diurnal variation in each of the surface energy balance components compared well with measured values. Model results were used to estimate total ET from the watershed as a basis for a complete water budget of the watershed. Pierson, 1991Pierson, , 1996, snowmelt and soil freezing [Flerchinger and Saxton, 1989;Flerchinger and Hanson, 1989;Flerchinger et al., 1994a;Hayhoe, 1994], and evaporation. However, the ability of the model to simulate ET and the entire surface energy balance has never been adequately tested. The primary purpose of this paper was to test the ability of the model to simulate the temporal surface energy balance of three types of vegetation across a small semiarid watershed. A secondary objective was to obtain an estimate of total ET for the watershed to be used in computing a water balance for the watershed. Numerous studies have been conducted to test various aspects of the SHAW model, including variability of soil temperature and moisture due to vegetation effects [Flerchinger and
Models used to simulate plant growth and insect development on rangeiands often assume that soil temperature is homogeneous over the entire area of interest. This simplifying assumption is made because few data are available on the magnitude and structure of the spatial variability of soil temperature within rangeland communities. The intluence of sagebrush on the spatial variability and diurnal fluctuations of near-surface soil temperature was examined within a sagebrush-grass plant community. Hourly soil temperatures were measured at l-, 5-, and NJ-cm depths at 38-cm intervals along a 12.3-m north-south transect over a 6-day period in March, 1989. Both classical and geostatistical techniques were used to quantify and model the magnitude and structure of the spatial and temporal variability. Maximum soil temperatures at the l-cm depth varied from 7 to 23" C under sagebrush and bare interspace, respectively. Periodic spatial patterns in soil temperature were found for all measured depths with a wavelength of periodicity approximately equal to the separation distance between sagebrush plants along the transect. Diurnal variability in near surface soil temperature was much greater in interspace areas compared to under sagebrush plants. The amplitude of diurnal variability in soil temperature at the l-cm depth under sagebrush was similar to the amplitude of the diurnal variability at the 18-cm depth within the interspaces.
Highlight: Nutrient deficiency, primarily nitrogen (N), is a major plant growth-limiting factor on northern Great Plains rangelands. Applications of 30 to 50 lb N/acre/year have commonly doubled forage production with an N-use efficiency of about 20 lb dry matter/lb N applied, or in grazing situations about 1 lb beef/lb N applied. Range fertilization can also increase water-use 'efficiency and improve forage quality and palatability. With applications of 50 lb N/acre/year or less, changes in species composition are gradual and can largely be controlled by timing of fertilizer applications and by season and intensity of grazing. Drastic changes in species composition are usually limited to applications greater than 15 0 lb N/acre/year. The northern Great Plains as described by Lodge (1970) covers approximately 400,000 square miles. Its general boundaries are the Rocky Mountains from the Wyoming-Colorado border north to Edmonton, Alberta, Canada, thence southeast to the intersection of the 98th meridian and the U.S.-Canadian border, thence south along the 98th meridian to the 41st parallel, and thence west to the Rocky Mountains. Native range accounts for approximately 70% of the land area and is primarily of the mixed prairie types with some tall grass prairie on the eastern extreme and some short grass prairie on the southwestern extreme. The Chestnut, Brown, and Dark Brown soils comprise the major soil groups within the area. The northern Great Plains has a climate which is generally considered harsh and extreme. Precipitation is irregular and often in short supply. In the east, annual precipitation averages more than 20 inches and in the north and west, as little as 10 inches per year. Among-year variation is even greater, particularly the seasonal precipitation, which often varies 100% or more. However, precipitation patterns within the northern Great Plains affect the vegetation more than does the total amount. About three-fourths of the annual precipitation occurs during the 6-month growing season (April through September) and about half during May, June, and July. Average growing season of the northern Great Plains decreases from 160 to 116 days going from south to north. Livestock production is a major segment of the economy in the northern Great Plains. Lodge (1970) estimated that this area supports some 12 million animal units of beef cattle and sheep. However, increasing demands for wheat and other cultivated crop products have resulted in the conversion of large acreages of range to crop land. Thus, to meet the The author is range scientist,
During the IO-year study, herbage production on an unfertilized, mixed prairie range site in eastern Montana averaged 1,047 kg/ha and ranged from 720 to 1,321 kg/ha. Elimination of nitrogen (N) and phosphorus (P) deficiencies by fertilizing increased herbage yields an average of 114% (ranging from a low of 32% in a "dry" year to a high of 218% in a "wet" year). Nitrogen was the major growth-limiting plant nutrient with measurable responses to P occurring only when N was nonlimiting. Single high-rate applications were about equal to annual N applications when compared on an annual rate equivalent basis. Species composition varied as much among years as among fertilizer treatments. At N rates of 336 kg/ha or less, cool-season grasses increased in about the same proportion as did forbs and shrubs, maintaining a relatively constant composition of the major species groups. On unfertilized plots, herbage yields and water use reached maximum values of about 1,250 kg/ha and 265 mm, respectively, regardless of further increases in available water. Unfertilized plots produced an average of 2.60 kg/ha for each 1 mm of precipitation received as compared with 5.81 kg/ha on fertilized plots.
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