Richard H. Cuenca

Evaluation of the evaporative fraction for parameterization of the surface energy balance

The evaporative fraction is a ratio of latent heat flux to the sum of latent and sensible heat fluxes. It has been used to characterize the energy partition over land surfaces and has potential for inferring daily energy balance information based on midday remote sensing measurements. The HAPEX-MOBILHY program SAMER system provided surface energy balance data over a range of agricultural crops and soil types. Data from this large-scale field experiment was analyzed to study the behavior and daylight stability of the evaporative fraction in both ideal and general meteorological conditions. Strong linear relations were found to exist between the midday evaporative fraction and the daylight evaporative fraction. Statistical tests, however, rejected the hypothesis that the two quantities were equal. Relations between the evaporative fraction and surface soil moisture as well as soil moisture over the complete root zone were explored, but no correlation was identified.

Land–Atmosphere Interaction Research, Early Results, and Opportunities in the Walnut River Watershed in Southeast Kansas: CASES and ABLE

Abstract This paper describes the development of the Cooperative Atmosphere Surface Exchange Study (CASES), its synergism with the development of the Atmosphere Boundary Layer Experiments (ABLE) and related efforts, CASES field programs, some early results, and future plans and opportunities. CASES is a grassroots multidisciplinary effort to study the interaction of the lower atmosphere with the land surface, the subsurface, and vegetation over timescales ranging from nearly instantaneous to years. CASES scientists developed a consensus that observations should be taken in a watershed between 50 and 100 km across; practical considerations led to an approach combining long—term data collection with episodic intensive field campaigns addressing specific objectives that should always include improvement of the design of the long—term instrumentation. In 1997, long—term measurements were initiated in the Walnut River Watershed east of Wichita, Kansas. Argonne National Laboratory started setting up the ABLE ar...

Soil water balance in a boreal forest

Measurements of root zone soil water content and soil hydraulic properties at the flux tower sites were conducted during the course of the Boreal Ecosystem-Atmosphere Study (BOREAS) experiment. Instrumentation included neutron probe, time domain reflectometry (TDR), and tension infiltrometer. Several methods of data visualization were employed to demonstrate fluctuations in soil water content with depth and time during the intensive field campaigns (IFCs). These methods started with two-dimensional plots of soil water content as a function of time and depth and evolved into the construction of three-dimensional soil water prisms. The prisms were constructed by using cubic-spline (vertical z-direction) and linear (horizontal x-direction) interpolations for soil water content and linear interpolations along the time (y direction) axis. The prisms allow for animation along any plane or combination of planes to demonstrate the evolution of soil water conditions due to evapotranspiration, drainage, and precipitation. Soil hydraulic properties were determined at the flux tower sites based on analysis of in situ tension infiltrometer tests and soil water retention data from laboratory analysis of soil cores. Results from this analysis are saturated hydraulic conductivity and fitting parameters for the van Genuchten soil water retention function and Mualem hydraulic conductivity function at each flux tower site. These functions are required for physically based simulation models of soil water dynamics, soil water balance, and the interaction of the soil profile with the atmospheric boundary layer. Examples of cumulative evapotranspiration and drainage calculated from the soil water balance are presented and compared with flux tower measurements.

Estimating Subcanopy Soil Moisture with RADAR

The subcanopy soil moisture of a boreal old jack pine forest stand is estimated using polarimetric L and P band airborne synthetic aperture radar (AIRSAR) data. Model simulations have shown that for this stand the principal scattering mechanism responsible for radar backscatter is the double-bounce mechanism between the tree trunks and the ground. The data to be used here were acquired during five flights from June to September 1994 as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) project. The dielectric constants, or equivalently moisture contents, of the trunks and soil can change significantly during this period. To estimate these dynamic unknowns, parametric models of radar backscatter for the double-bounce mechanism are developed using a series of simulations of a numerical forest scattering model. A nonlinear optimization procedure is used to estimate the dielectric constants. Ground measurements of soil and trunk moisture content are used to validate the results. The trunk moisture content measurements are used to gain confidence that the respective estimation results are accurate enough not to corrupt the soil moisture estimation, which is the main focus of this paper. After conversion of the trunk moisture measurements to dielectric constants it is found that the estimated values are within 14% of the measurements. Owing to possible calibration uncertainties in the soil moisture measurements on the ground as well as in AIRSAR data, the variations rather than the absolute levels of the estimated soil moisture are considered. The results indicate that the estimated variations closely track the measurements. The worst case average estimated change differs by <1% volumetric soil moisture from that measured on the ground.

Physical basis for a time series model of soil water content

A first-order autoregressive Markovian model AR(1) is formulated on the basis of the hydrologic budget and soil water transport equation. The model predictions compared well with neutron probe measurements of soil moisture content, and the statistical moments were conserved. The applied water events were white noise in structure, and the random shocks generated from the flow dynamics simplifications have a statistical mean of zero and were uncorrelated for all time lags. The derived AR(1) model parameter is used to compute the mean diffusivity of the soil, which is in agreement with reported lab measurements and field estimates obtained from cumulative evaporation measurements made with two large lysimeters.

VARIATION IN SOIL PARAMETERS: IMPLICATIONS FOR MODELING SURFACE FLUXES AND ATMOSPHERIC BOUNDARY-LAYER DEVELOPMENT

Soil texture can be heterogeneous; however for land surface-atmospheric modeling purposes, it is often considered homogeneous at a particular point and described by empirical equations which have been formulated to describe ‘average’ hydraulic and thermodynamic processes in the soil. Large deviations in the variables and coefficients used in these empirical equations have been previously documented. One of the coefficients is varied by plus-and-minus one standard deviation about its mean, and tested in a coupled atmospheric-plant-soil model. Results of model simulations show that the effects on surface fluxes and boundary-layer development are larges for dry to moderate values of soil moisture, particularly for bare soil conditions.

Soil measurements during HAPEX-Sahel intensive observation period

This article describes measurements made at each site and for each vegetation cover as part of the soils program for the HAPEX-Sahel regional scale experiment. The measurements were based on an initial sampling scheme and included profile soil water content, surface soil water content, soil water potential, infiltration rates, additional measurements on core samples, and grain size analysis. The measurements were used to categorize the state of the surface and profile soil water regimes during the experiment and to derive functional relationships for the soil water characteristic curve, unsaturated hydraulic conductivity function, and infiltration function. Sample results for different supersites and different vegetation covers are presented showing soil water profiles and total soil water storage on days corresponding to the experimental ‘Golden Days’. Sample results are also presented for spatial and temporal distribution of surface moisture content and infiltration tests. The results demonstrate that the major experimental objective of monitoring the supersites during the most rapid vegetative growth stage with the largest change of the surface energy balance following the rainy season was very nearly achieved. Separation of the effects of probable root activity and drainage of the soil profile is possible. The potential for localized advection between the bare soil and vegetation strips of the tiger bush sites is demonstrated

Daytime variation of sensible heat flux estimated by the bulk aerodynamic method over a grass canopy

Abstract A study was performed to evaluate the bulk aerodynamic method for estimating daytime variation of sensible heat flux ( H A ) for a wetted and non-wetted grass canopy under clear and cloudy days. The aerodynamic resistance applied was corrected for atmospheric stability using the Oregon State University One-Dimensional Planetary Boundary Layer model stability function. The performance of the bulk aerodynamic method was tested with indirect measurements of sensible heat flux ( H B ) derived from the Bowen ratio energy balance method on 20-min time intervals. Results indicate that there was a good agreement between H B and H A with a coefficient of correlation of 0.93 and slope of the regression line through the origin of 0.96.

NCAR/CU Surface, Soil, and Vegetation Observations during the International H2O Project 2002 Field Campaign

Abstract The May–June 2002 International H2O Project was held in the U.S. Southern Great Plains to determine ways that moisture data could be collected and utilized in numerical forecast models most effectively. We describe the surface and boundary layer components, and indicate how the data can be acquired. These data document the eddy transport of heat and water vapor from the surface to the atmosphere (in terms of sensible heat flux H and latent heat flux LE), as well as radiative, atmospheric, soil, and vegetative factors that affect it, so that the moisture and heat supply to the atmosphere can be related to surface properties both for observational studies and tests of land surface models. The surface dataset was collected at 10 surface flux towers at locations representing the major types of land cover and extending from southeast Kansas to the Oklahoma Panhandle. At each location, the components of the surface energy budget (H, LE, net radiation, and soil heat flux) are documented each half-hour, ...

Special issue on evapotranspiration measurement and modeling

Water availability for irrigation throughout the world has been reduced in recent years due to a combination of frequent droughts and competition for water resources among agricultural, industrial, and urban users. In addition, some major agricultural areas face moderate to significant reductions of rainfall, or changes in timing of stream flow due to changes in timing of snowmelt, as a result of global climate change. Under such conditions, sophisticated irrigation water management will be required to optimize water use efficiency and maintain sufficient levels of crop productivity and quality. A key factor to achieve these targets is the estimation of actual evapotranspiration (ET). Accurate determination of ET can be a viable tool in better utilization of water resources through well-designed irrigation management programs. Reliable estimates of ET are also vital to develop criteria for in-season irrigation management, water resource allocation, long-term estimates of water supply, demand and use, design and management of water resources infrastructure, and evaluation of the effect of land use and management changes on the water balance. ET is commonly calculated using grass or alfalfareference evapotranspiration (ETo) multiplied by grass or alfalfa-reference-based crop-specific coefficients (Kc). The Penman–Monteith combination equation is widely accepted as the best-performing method for reference evapotranspiration estimates from a well-watered hypothetical grass or alfalfa surface having a fixed crop height, albedo, and surface canopy resistance. The Kc is basically the ratio of ET to ETo where ET can be measured using a lysimeter, soil water balance approach, eddy covariance method, Bowen ratio energy balance system, or surface renewal method. Advances over the last two to three decades in instrumentation, data acquisition systems, remote data access, and the off-the-shelf availability of aforementioned ET measurement tools have significantly enhanced our understanding of ET and its relation to microclimatic conditions. Advances also enabled the availability and affordability of data for practitioners for use in irrigation management. While the reference ET and Kc approach provides a simple and convenient way to estimate crop water requirements for a variety of crops and climatic conditions, a major uncertainty in this approach is that many Kc values reported in the literature are empirical and often not adapted to local conditions. This is due to the fact that ratios of ET to ETo depend on nonlinear interactions of soil, crop and atmospheric conditions, and irrigation management practices. This consideration is especially Communicated by R. Evans.