Estimating Transpiration from Turfgrass Using Stomatal Conductance Values Derived from Infrared Thermometry

Infrared thermometry provides accurate measurements of plant canopy temperature, which, along with basic weather variables, allows estimation of canopy stomatal conductance to water vapor flux (gc) and transpiration. Using this method we compared modeled estimates of transpiration (CONDT) with evapotranspiration (ET) measurements from nearby microlysimeters (LYSET) in tall fescue (Schedonorus arundinaceus Schreb.) turfgrass. Results indicated transpiration may be reliably estimated via calculation of gc in turfgrass

The future of evapotranspiration : global requirements for ecosystem functioning, carbon and climate feedbacks, agricultural management, and water resources

The fate of the terrestrial biosphere is highly uncertain given recent and projected changes in climate. This is especially acute for impacts associated with changes in drought frequency and intensity on the distribution and timing of water availability. The development of effective adaptation strategies for these emerging threats to food and water security are compromised by limitations in our understanding of how natural and managed ecosystems are responding to changing hydrological and climatological regimes. This information gap is exacerbated by insufficient monitoring capabilities from local to global scales. Here, we describe how evapotranspiration (ET) represents the key variable in linking ecosystem functioning, carbon and climate feedbacks, agricultural management, and water resources, and highlight both the outstanding science and applications questions and the actions, especially from a space-based perspective, necessary to advance them

Evaluation of Two New Net Radiometers

Net radiation is a key component to the surface energy balance, but it is difficult and expensive to measure accurately. Two new net radiometer models (Hukseflux NR01 and Kipp & Zonen CNR 2) have been released in the past year. We evaluated and compared these models to each other, a Kipp and Zonen model CNR 1 net radiometer, and to two less expensive, older model net radiometers (Kipp & Zonen NR-Lite and REBS Q*7.1). Hourly averages and daily totals (over the course of the study; 33 days) from three replicate sensors of the two new net radiometers compared quite well to the CNR 1 radiometer. The difference was generally less than +/- 5 %. Three replicates of the two older model net radiometers did not agree as well with the newer models, particulary at night, with differences generally less than +/- 10 % during the day and +/- 20 % at night. Our data matched what others (Cobos and Baker, 2003; Brotzge and Duchon, 2000) have shown for these older radiometers. Our findings indicate that accuracy increases with increasing cost. Accurate net radiation measurements depend on proper placement of the sensor, proper leveling, and routine maintenance to keep the sensing surfaces clean

Micro-Structural and Phase Configuration Effects Determining Water Content: Dielectric Relationships of Aggregated Porous Media

Many porous media in which we determine water content are aggregated and characterized by a dual-porosity pore network, composed of interaggregate pores and intra-aggregate pores. This paper reports sample-scale permittivity measurements made in four stable aggregate media with dual porosity. Results indicate two distinct dielectric responses depending on whether the aggregates are surrounded by water or air. We relate transitions in the permittivity response to the water retention characteristic (WRC), showing that after the interaggregate pores have drained, the slope of the water content–permittivity relationship is significantly reduced (permittivity values ranging from 5 to 7). The hydraulic critical water content (θhc) is defined as the point where all the interaggregate pores are air filled and all the intra-aggregate pores are water saturated and is determined from the WRC. The dielectric critical water content (θdc) is defined as the point where a slope change in the measured water content–permittivity relationship occurs. A two-step model is presented and designed to capture the physical characteristics of the permittivity response to drainage. Measurements of θhc and θdc reveal a separation in these two values. The difference is considered to be a function of the connectivity of the intra-aggregate pore network. A connectivity ratio is defined as 1-(θdc/θhc), where values close to 0 indicate low connectivity between the intra-aggregate pores and values tending to 1 indicate a high level of connectivity. Results from this work indicate that the reduced permittivity response measured in the water content–permittivity relationship is due to microstructure and phase configuration and not to “bound” water