Douglas R. Cobos

Diel and seasonal variation in CO2 flux of irrigated rice

Abstract Rice is a primary food source for half the world’s population, but little is known of how temporal changes in the field-scale physical environment affect carbon dioxide exchange rate (CER), biomass accumulation, and crop yield. Experiments were conducted in 1998 and 1999 in a commercial field near El Campo, TX, to evaluate interactions between CER and the physical environment. Tower-based conditional sampling was used to measure CER. Environmental parameters such as photosynthetically active radiation (PAR), net radiation, and temperature were measured along with CER. Whole-plant biomass was also collected throughout both seasons. Fluctuations in diel CER were correlated with changes in PAR, while season-long trends in CER were associated with changes in leaf area index and stage of development. Crop yield was found to be directly related to total carbon-dioxide exchange after heading, and may have been affected by environmental conditions at anthesis, such as temperature and wind speed, or leaf nitrogen status, both of which differed considerably between the two seasons. Data showed a positive correlation between biomass accumulation and cumulative CER for both years of the study.

In Situ Measurement of Soil Heat Flux with the Gradient Method

The use of soil heat flux as a critical component of the surface energy balance is routine; however, its accurate quantification is not. The direct measurement of soil heat flux is generally accomplished with soil heat flux plates. However, the presence of heat flux plates causes perturbations in heat and fluid flow in the soil that may give rise to measurement errors. We describe here the direct measurement of soil heat flux with the gradient technique using a tri-needle instrument, where soil heat flux is the product of the soil thermal conductivity measured by transient heating of the center needle and the soil temperature gradient measured between the outer needles. Soil heat fluxes measured this way were compared with those obtained from commercially available soil heat flux plates. Laboratory trials revealed good precision and accuracy (error generally <5%) in measurements of thermal conductivity and soil heat flux with the heated needle technique in a medium-textured sand, but exposed occasional errors in the accurate determination of thermal conductivity arising from poor sensor-soil thermal contact in a coarser medium. Extended field data in a finer-textured soil showed good agreement among needle sensors and heat flux plates, and surprisingly did not reveal errors associated with fluid blockage by the heat flux plates. Our results indicate that the measurement of soil heat flux with the gradient technique with needle sensors is a viable alternative to heat flux plates, and may improve absolute accuracy of this measurement.

Tower-based conditional sampling for measuring ecosystem-scale carbon dioxide exchange in coastal wetlands

Long-term measurements of CO2 exchange between coastal wetlands and the atmosphere are necessary to improve our understanding of the role these ecosystems play in the global carbon cycle, and the response of these systems to environmental change. We conducted research to adapt and evaluate tower-based conditional sampling as a method for measuring net CO2 exchange (NCE) at the ecosystem scale on a continuous basis. With conditional sampling, NCE is determined from the product of the standard deviation of vertical wind velocity, the difference in CO2 concentration between updrafts and downdrafts in the constant flux portion of the boundary layer above the surface, and an empirical coefficient. We constructed a system that used a sonic anemometer to measure vertical wind velocity (w) and control a high-speed three-way valve that diverted air from updrafts and downdrafts into separate sample lines, depending on the direction ofw. an infrared gas analyzer was used to measure the concentration difference. The conditional sampling system was installed and tested in a marsh in the Nueces River Delta near Corpus Christi, Texas, as part of a long-term study of effects of freshwater inflow on CO2 flux. System accuracy was evaluated by comparing conditional sampling measurements of water vapor flux with independent estimates obtained with the Bowen ratio method. Average daily flux estimates for the two methods agreed to within 13%. Measurements showed that freshwater inflow due to flooding of the Nueces River increased NCE by increasing CO2 assimilation and decreasing CO2 efflux. Over a 65-d period, daily NCE varied from a maximum gain of 0.16 mol CO2 m−2 d−1 during flooding to a maximum loss of −0.14 mol CO2 m−2 d−1 when the marsh dried. Our study showed that conditional sampling was well suited for quantifying CO2 exchange in coastal wetlands on a diel, daily, and seasonal basis.

Direct Calculation of Thermodynamic Wet-Bulb Temperature as a Function of Pressure and Elevation

A simple analytical method was developed for directly calculating the thermodynamic wet-bulb temperature from air temperature and the vapor pressure (or relative humidity) at elevations up to 4500 m above MSL was developed. This methodology was based on the fact that the wet-bulb temperature can be closely approximated by a second-order polynomial in both the positive and negative ranges in ambient air temperature. The method in this study builds upon this understanding and provides results for the negative range of air temperatures (2178 to 08C), so that the maximum observed error in this area is equal to or smaller than 20.178C. For temperatures

A pragmatic, automated approach for retroactive calibration of soil moisture sensors using a two-step, soil-specific correction

08C, wet-bulb temperature accuracy was 60.658C, and larger errors correspondedto very high temperatures (Ta

Constructing Fast, Accurate Soil Water Characteristic Curves by Combining the Wind/Schindler and Vapor Pressure Techniques

398C) and/or very high or low relative humidities(5%,RH,10% or RH . 98%). The mean absolute error and the root-mean-square error were 0.158 and 0.28C, respectively.

Photosynthetic response, survival, and growth of three ponderosa pine stocktypes under water stress enhanced by vegetative competition

Individual laboratory calibration of many soil water content sensors is unpractical.We applied sensor-specific calibrations based on soil properties at insertion sites.This method is a retroactive approach for acquired sensor data.Produced accurate sensor values in a network installed across diverse soil profiles. Soil moisture sensors are increasingly deployed in sensor networks for both agronomic research and precision agriculture. Soil-specific calibration improves the accuracy of soil water content sensors, but laboratory calibration of individual sensors is not practical for networks installed across heterogeneous settings. Using daily water content readings collected from a sensor network (42 locations5 depths=210 sensors) installed at the Cook Agronomy Farm (CAF) near Pullman, Washington, we developed an automated calibration approach that can be applied to individual sensors after installation. As a first step, we converted sensor-based estimates of apparent dielectric permittivity to volumetric water content using three different calibration equations (Topp equation, CAF laboratory calibration, and the complex refractive index model, or CRIM). In a second, re-calibration step, we used two pedotransfer functions based upon particle size fractions and/or bulk density to estimate water content at wilting point, field capacity, and saturation at each sensor insertion point. Using an automated routine, we extracted the same three reference points, when present, from each sensors record, and then bias-corrected and re-scaled the sensor data to match the estimated reference points. Based on validation with field-collected cores, the Topp equation provided the most accurate calibration with an RMSE of 0.074m3m3, but automated re-calibration with a local pedotransfer function outperformed any of the calibrations alone, yielding a network-wide RMSE of 0.055m3m3. The initial calibration equation used in the first step was irrelevant when the re-calibration was applied. After correcting for the reference core measurement error of 0.026m3m3 used for calibration and validation, the error of the sensors alone (RMSEadj) was computed as 0.049m3m3. Sixty-five percent of individual sensors exhibited re-calibration errors less than or equal to the network RMSEadj. The incorporation of soil physical information at sensor installation sites, applied retroactively via an automated routine to in situ soil water content sensors, substantially improved network sensor accuracy.

Frequency, electrical conductivity and temperature analysis of a low-cost capacitance soil moisture sensor

The measurement of suction in unsaturated soils is essential in many areas of geotechnical engineering like modeling hydraulic properties or analyzing shrink-swell characteristics from the soil water characteristic curve (SWCC). For many years, filter paper and axis translation made the majority of the SWCCs, but these techniques are time consuming and inaccurate. The vapor pressure method (VPM) overcomes these, but past instruments lacked the resolution in the critical 100 to 1000 kPa range. Recently, technological advances have made it possible to measure soil suction up to 50 kPa. In addition, a new, automated method (Wind/Schindler; WSM) provides the wet end of the SWCC. Several water retention curves were run for a variety of soil types. Characteristic curves crossed over each other smoothly. The VPM method combined with the WSM provide both speed and accuracy to the measurement of soil suction, construction of SWCC, and soil hydraulic properties evaluation.

Carbon dioxide exchange in a high marsh on the Texas Gulf Coast: effects of freshwater availability

Selecting the proper stock type for reforestation on dry sites can be critical for the long-term survival and growth of seedlings. In this study, we use a novel approach to understand stock type selection on a site where drought was induced with vegetative competition. Three ponderosa pine (Pinus ponderosa Lawson & C. Lawson var. ponderosa C. Lawson) seedling stock types were planted in the field and subjected to three levels of competition. Winter wheat (Triticum aestivum L. em.) was sown in three densities (0, 150, and 300 plants·m-2) and was successfully used as a model competitor to create drought conditions. High rates of net photosynthesis (A) indicated that seedlings with adequate soil moisture and without vegetative competition were established within three weeks. Conversely, low A, low soil moisture, and low predawn water potential measurements indicated that seedlings planted with vegetative competition were moisture-stressed and not established. Drought conditions created by the wheat caused 100% mortality among smaller stock types, whereas the largest stock type had a 63%-75% mortality rate.