Mark A. Weltz

Combining the Penman-Monteith equation with measurements of surface temperature and reflectance to estimate evaporation rates of semiarid grassland

The Penman-Monteith equation is useful for computing evaporation rates of uniform surfaces, such as dense vegetation or bare soil. This equation becomes less useful for evaluation of evaporation rates at the regional scale, where surfaces are generally characterized by a patchy combination of vegetation and soil. This is particularly true in the arid and semi-arid regions of the world. The approach proposed here is an attempt to use remotely-sensed measurements of surface reflectance and temperature to allow application of the Penman-Monteith theory to partiallyvegetated fields without a-priori knowledge of the percent vegetation cover and canopy resistance. Basically, the Penman-Monteith equation was combined with the energy balance equation to estimate the surface temperature CT,) associated with four states: surfaces characterized by full-cover vegetation and bare soil with evaporation rates at potential and zero. Then, linear interpolations between T, values computed for full-cover and bare soil conditions were used to provide information at intermediate states based on measurements of actual surface reflectance and temperature. The approach was first tested using ground-based measurements of surface reflectance and temperature at a rangeland site; the results compared well with on-site measure

Comparison of laser and field measurements of vegetation height and canopy cover

Distribution of vegetation properties is fundamental for understanding vegetation patterns and characteristics, improving estimates of infiltration, evapotranspiration, and soil erosion. A laser altimeter mounted in a small airplane was used to measure surface patterns of the landscape on the U.S. Department of Agricultures Agricultural Research Service Walnut Gulch experimental watershed near Tombstone, Arizona. The airborne laser altimeter is a pulsed gallium-arsenide diode laser, transmitting and receiving 4000 pulses per second at a wavelength of 0.904 μm. The laser has a 1-mrad field of view and is designed to have a vertical recording precision of 0.05 m on a single measurement. Aircraft altitude varied between 100 and 300 m for the flights. Digital data from the laser were collected with a portable computer and analyzed to provide information on changes in vegetation height, spatial patterns, and patchiness of vegetation cover. The laser-measured vegetation properties of plant height and canopy cover (>0.3 m) were not significantly different than field measurements made using the line-intercept transect method at seven of the eight sites evaluated. Although the laser measurements of canopy height were not significantly different from the ground measurements, the laser consistently overestimated canopy cover less than 0.3 m in height and underestimated canopy cover greater than 0.5 m. New techniques to discriminate the background noise in the laser return signal in sparsely populated shrub communities are necessary before this technique will be fully useful in estimating canopy cover on rangelands. These studies indicate the potential of airborne laser to measure vegetation patterns quickly and quantitatively. The laser also has the ability to separate and map distinctly different plant communities.

An interdisciplinary field study of the energy and water fluxes in the atmosphere−biosphere system over semiarid rangelands : description and some preliminary results

Abstract Arid and semiarid rangelands comprise a significant portion of the earths land surface. Yet little is known about the effects of temporal and spatial changes in surface soil moisture on the hydrologic cycle, energy balance, and the feedbacks to the atmosphere via thermal forcing over such environments. Understanding this interrelationship is crucial for evaluating the role of the hydrologic cycle in surface-atmosphere interactions. This study focuses on the utility of remote sensing to provide measurements of surface soil moisture, surface albedo, vegetation biomass, and temperature at different spatial and temporal scales. Remote-sensing measurements may provide the only practical means of estimating some of the more important factors controlling land surface processes over large areas. Consequently, the use of remotely sensed information in biophysical and geophysical models greatly enhances their ability to compute fluxes at catchment and regional scales on a routine basis. However, model cal...

Remote Sensing for Grassland Management in the Arid Southwest

Abstract We surveyed a group of rangeland managers in the Southwest about vegetation monitoring needs on grassland. Based on their responses, the objective of the RANGES (Rangeland Analysis Utilizing Geospatial Information Science) project was defined to be the accurate conversion of remotely sensed data (satellite imagery) to quantitative estimates of total (green and senescent) standing cover and biomass on grasslands and semidesert grasslands. Although remote sensing has been used to estimate green vegetation cover, in arid grasslands herbaceous vegetation is senescent much of the year and is not detected by current remote sensing techniques. We developed a ground truth protocol compatible with both range management requirements and Landsats 30 m resolution imagery. The resulting ground-truth data were then used to develop image processing algorithms that quantified total herbaceous vegetation cover, height, and biomass. Cover was calculated based on a newly developed Soil Adjusted Total Vegetation Index (SATVI), and height and biomass were estimated based on reflectance in the near infrared (NIR) band. Comparison of the remotely sensed estimates with independent ground measurements produced r2 values of 0.80, 0.85, and 0.77 and Nash Sutcliffe values of 0.78, 0.70, and 0.77 for the cover, plant height, and biomass, respectively. The approach for estimating plant height and biomass did not work for sites where forbs comprised more than 30% of total vegetative cover. The ground reconnaissance protocol and image processing techniques together offer land managers accurate and timely methods for monitoring extensive grasslands. The time-consuming requirement to collect concurrent data in the field for each image implies a need to share the high fixed costs of processing an image across multiple users to reduce the costs for individual rangeland managers.

The first five years of the Conservation Effects Assessment Project

The Conservation Effects Assessment Project (CEAP) was initiated by the USDA Natural Resources Conservation Service (NRCS), Agricultural Research Service (ARS), and Cooperative State Research, Education, and Extension Service (CSREES) in response to a general call for better accountability of how society would benefit from the 2002 farm bills substantial increase in conservation program funding (Mausbach and Dedrick 2004). The original goals of CEAP were to establish the scientific understanding of the effects of conservation practices at the watershed scale and to estimate conservation impacts and benefits for reporting at the national and regional levels. Other federal agencies and nongovernmental organizations with conservation and natural resource interests are currently partners in various CEAP activities, often through jointly funded research projects. CEAP activities are organized into three interconnected efforts: Bibliographies, literature reviews, and a scientific workshop to establish what is known about the environmental effects of conservation practices at field and watershed scales, and what kinds of research and data collection are needed to assess conservation practice benefits. Watershed assessment studies to provide in-depth quantification of water quality and soil quality impacts of conservation practices at the local level and to provide insight on what practices are needed…

Evaluation of hydrologic parameters in a semiarid rangeland using remotely sensed spectral data

A study was conducted to determine the relation between remotely sensed spectral data and measurements of vegetation-related hydrologic parameters in a semiarid rangeland in southeast Arizona. Throughout the measurement periods, ranging from June to September 1990, eight sites in the U.S. Department of Agricultures Agricultural Research Service Walnut Gulch experimental watershed were monitored for water and energy fluxes and other meteorological and biological parameters. Corresponding spectral data were acquired with ground-based radiometers, low-altitude aircraft-mounted instruments, and Landsat thematic mapper sensors. Spectral indices were derived from measurements of surface reflectance, based on their response to variations in hydrologic parameters and sensitivity to unrelated variables, such as solar zenith angle and soil differences. A soil-adjusted vegetation index, SAVI (derived from red and NIR reflectance factors), was found to be highly correlated with the temporal changes in vegetation cover and biomass, but less successful in discriminating spatial differences in cover and biomass across the watershed. Significant relations were found between the surface-air temperature (Ts-Ta) difference and measurements of soil moisture content, though the shape differed from that previously published for bare soil. The relation between daily evaporation rate and measurements of (Ts-Ta) and daily net radiation was similar to that derived previously for irrigated pasture and dryland shortgrass in France but differed from that derived for irrigated wheat. These results emphasized the strengths and limitations of the use of spectral data for estimation of hydrologic characteristics of sparsely vegetated sites and suggested a need for reevaluation of common empirical relations between remotely sensed measurements and surface characteristics for application to rangeland areas.

First order surface roughness correction of active microwave observations for estimating soil moisture

Surface roughness has a significant effect on the relationship between radar backscatter and soil moisture. In order to use existing radar satellite data for soil moisture, roughness effects must be corrected. A technique is presented that utilizes the data bases from soil erosion studies and soil moisture remote sensing investigations to provide first order estimates of the roughness parameters.

A Rangeland Hydrology and Erosion Model

Soil loss rates on rangelands are considered one of the few quantitative indicators for assessing rangeland health and conservation practice effectiveness. An erosion model to predict soil loss specific for rangeland applications is needed because existing erosion models were developed from croplands where the hydrologic and erosion processes are different, largely due to much higher levels of heterogeneity in soil and plant properties at the plot scale and the consolidated nature of the soils. The Rangeland Hydrology and Erosion Model (RHEM) was designed to fill that need. RHEM is an event-based derivation of the WEPP model made by removing relationships developed specifically for croplands and incorporating new equations derived from rangeland data. RHEM represents erosion processes under disturbed and undisturbed rangeland conditions, it adopts a new splash erosion and thin sheet-flow transport equation developed from rangeland data, and it links the model hydrologic and erosion parameters with rangeland plant communities by providing a new system of parameter estimation equations based on 204 plots at 49 rangeland sites distributed across 15 western U.S. states. RHEM estimates runoff, erosion, and sediment delivery rates and volumes at the spatial scale of the hillslope and the temporal scale of a single rainfall event. Experiments were conducted to generate independent data for model evaluation, and the coefficients of determination (r2) for runoff and erosion predictions were 0.87 and 0.50, respectively, which indicates the ability of RHEM to provide reasonable runoff and soil loss prediction capabilities for rangeland management and research needs.

Push broom microwave radiometer observations of surface soil moisture in Monsoon '90

The push broom microwave radiometer (PBMR) was flown on six flights of the NASA C-130 to map the surface soil moisture over the U.S. Department of Agricultures Agricultural Research Service Walnut Gulch experimental watershed in southeastern Arizona. The PBMR operates at a wavelength of 21 cm and has four horizontally polarized beams which cover a swath of 1.2 times the aircraft altitude. By flying a series of parallel flight lines it was possible to map the microwave brightness temperature (TB), and thus the soil moisture, over a large area. In this case the area was approximately 8 by 20 km. The moisture conditions ranged from very dry, 15%, after a heavy rain. The rain amounts ranged from less than 10 mm to more than 50 mm over the area mapped with the PBMR. With the PBMR we were able to observe the spatial variations of the rain amounts and the temporal variation as the soil dried. The TB values were registered to a Universal Transverse Mercator grid so that they could be compared to the rain gage readings and to the ground measurements of soil moisture in the 0- to 5-cm layer. The decreases in TB were well correlated with the rainfall amounts, R2 = 0.9, and the comparison of Tg with soil moisture was also good with an R2 of about 0.8. For the latter, there was some dependence of the relation on location, which may be due to soil or vegetation variations over the area mapped. The application of these data to runoff forecasts and flux estimates will be discussed.

Measuring Canopy Structure with an Airborne Laser Altimeter

Quantification of vegetation patterns and properties is needed to determine their role on the landscape and to develop management plans to conserve our natural resources. Quantifying vegetation patterns from the ground, or by using aerial photography or satellite imagery is difficult, time consuming, and often expensive. Digital data from an airborne laser altimeter offer an alternative method to quantify selected vegetation properties and patterns of forest and range vegetation. Airborne laser data found canopy heights varied from 2 to 6 m within even-aged pine forests. Maximum canopy heights measured with the laser altimeter were significantly correlated to measurements made with ground-based methods. Canopy shape could be used to distinguish deciduous and evergreen trees. In rangeland areas, vegetation heights, spatial patterns, and canopy cover measured with the laser altimeter were significantly related with field measurements. These studies demonstrate the potential of airborne laser data to measure canopy structure and properties for large areas quickly and quantitatively.