Shirley A. Kurc

Dynamics of evapotranspiration in semiarid grassland and shrubland ecosystems during the summer monsoon season, central New Mexico

higher at the grassland than at the shrubland by 20% or 70 W m � 2 because of differences in net radiation (Rn) and soil heat flux (G). At both sites, midday evaporative fraction and daily ET are strongly correlated with surface soil moisture (q0–5cm) but poorly correlated with water content at greater depths or averaged throughout the entire root zone. The sensitivity of EF to q0–5cm is 30% lower at the grassland site. The differences in Qa and EF cancel, yielding similar time series of ET at the two sites. Decreases in q0–5cm, ET, and EF following rainfall events are rapid: exponential time constants are less than 3 days. With the exception of the largest storms, infiltration following rainfall events only wets the top 10 cm of soil. Therefore the surface soil layer is the primary reservoir for water storage and source for ET during the monsoon season, suggesting that direct evaporation is a large component of ET. Given these results, predicting ET based on root zone–averaged soil moisture is inappropriate in the semiarid environments studied here. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 1833 Hydrology: Hydroclimatology; 1866 Hydrology: Soil moisture; 1878 Hydrology: Water/energy interactions; KEYWORDS: Bouteloua eriopoda, Bowen ratio, evapotranspiration, grassland, Larrea tridentata, shrubland Citation: Kurc, S. A., and E. E. Small (2004), Dynamics of evapotranspiration in semiarid grassland and shrubland ecosystems during the summer monsoon season, central New Mexico, Water Resour. Res., 40, W09305, doi:10.1029/2004WR003068.

Assessing interannual variation in MODIS-based estimates of gross primary production

Global estimates of terrestrial gross primary production (GPP) are now operationally produced from Moderate Resolution Imaging Spectrometer (MODIS) imagery at the 1-km spatial resolution and eight-day temporal resolution. In this study, MODIS GPP products were compared with ground-based GPP estimates over multiple years at three sites-a boreal conifer forest, a temperate deciduous forest, and a desert grassland. The ground-based estimates relied on measurements at eddy covariance flux towers, fine resolution remote sensing, and modeling. The MODIS GPP showed seasonal variation that was generally consistent with the in situ observations. The sign and magnitude of year-to-year variation in the MODIS products agreed with that of the ground observations at two of the three sites. Examination of the inputs to the MODIS GPP algorithm-notably the fraction of photosynthetically active radiation (FPAR) that is absorbed by the canopy), minimum temperature scalar, and vapor pressure deficit scalar-provided explanations for cases of disagreement between the MODIS and ground-based GPP estimates. Continued evaluation of interannual variation in MODIS products and related climate variables will aid in assessing potential biospheric feedbacks to climate change

Tight coupling between soil moisture and the surface radiation budget in semiarid environments: Implications for land-atmosphere interactions

Received 11 March 2002; revised 2 May 2003; accepted 20 June 2003; published 4 October 2003. [1] Observations are used to examine how soil moisture influences the surface radiation budget, ground heat flux, and available energy in semiarid environments. Defining this relationship is critical to understand interactions between the land surface and the atmosphere, in particular assessing if a feedback exists between soil moisture and rainfall anomalies. We use two summers of data collected from semiarid grassland and shrubland ecosystems in central New Mexico. The response of surface radiation budget components and other variables to soil moisture variations are quantified via linear regression. Then, the variations are scaled over the observed range of soil moisture (15% volumetric water content). The soil temperature is lower by >10� C when the surface soil is wet, compared to when the soil is dry. This temperature decrease results in a measured decrease of 85–100 W m � 2 in longwave radiation emitted at the surface. The increase in net longwave radiation is equal in magnitude because downward longwave radiation does not vary with soil moisture. The observed changes in net shortwave radiation are relatively minor (<10 W m � 2 ), as the surface albedo decreases by only 1.5% when soil is wet. Net radiation increases by an amount roughly equal to the decrease in emitted longwave radiation (� 85–100 W m � 2 ). Changes in ground heat flux are not detectable, given the noise in the data. Therefore the available energy, Qa, is higher by 80 W m � 2 when the soil is wet. This change is 22% of average Qa at the shrubland site and 19% at the grassland site. The observed soil moisture-induced Qa variations are large compared to other sources of Qa variability, so they should influence boundary layer moist static energy. However, the intervals during which soil moisture is high and therefore Rn and Qa are enhanced are short, on the order of several days. Therefore feedbacks to rainfall may be limited. Compared to other environments, the influence of soil moisture on Rn and Qa is likely greater in semiarid environments because soil moisture-induced fluctuations in evaporative fraction and surface temperature are relatively large. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 1833 Hydrology: Hydroclimatology; 1866 Hydrology: Soil moisture; 3322 Meteorology and Atmospheric Dynamics: Land/ atmosphere interactions; KEYWORDS: soil moisture, ground heat flux, soil temperature, net radiation, evapotranspiration, land-atmosphere interactions

Density-Dependent Ecohydrological Effects of Piñon–Juniper Woody Canopy Cover on Soil Microclimate and Potential Soil Evaporation

Abstract Many rangeland processes are driven by microclimate and associated ecohydrological dynamics. Most rangelands occur in drylands where evapotranspiration normally dominates the water budget. In these water-limited environments plants can influence abiotic and biotic processes by modifying microclimate factors such as soil temperature and potential soil evaporation. Previous studies have assessed spatial variation in microclimate and associated ecohydrological attributes within an ecosystem (e.g., under vs. between woody canopies) or across ecosystems (e.g., with differing amounts of woody canopy cover), but generally lacking are assessments accounting systematically for both, particularly for evergreen woody plants. Building on recently quantified trends in near-ground solar radiation associated with a piñon–juniper gradient spanning 5% to 65% woody canopy cover, we evaluated trends in soil temperature and associated estimates of potential soil evaporation as a function of amount of woody canopy cover for sites overall and for associated canopy vs. intercanopy locations. Quantified soil temperature trends decreased linearly with increasing woody canopy cover for intercanopy as well as canopy patches, indicating the coalescing influence of individual canopies on their neighboring areas. Notably, intercanopy locations within high-density (65%) woody canopy cover could be as much as ∼10°C cooler than intercanopy locations within low-density (5%) cover. Corresponding potential soil evaporation rates in intercanopies within high-density woody canopy cover was less than half that for intercanopies within low density. Our results highlight ecohydrological consequences of density-dependent shading by evergreen woody plants on soil temperature and potential soil evaporation and enable managers to rapidly estimate and compare approximate site microclimates after assessing amounts of woody canopy cover. Such predictions of microclimate have general utility for improving management of rangelands because they are a fundamental driver of many key processes, whether related to understory forage and herbaceous species or to wildlife habitat quality for game or nongame species.