Multisensor capacitance probes (MCAP) are an alternative to gravimetric or nuclear soil water content (θv, m3 m−3) measurements. Their θv measurements are more convenient than gravimetric, and don't carry the nuclear regulatory burdens. Previous studies noted potential salinity and temperature effects on MCAP θv determinations. Our objectives were to calibrate and verify MCAP θv measurement accuracy in two soil materials, two water salinities (1.3 and 11.3 dS m−1), and with diurnal temperature fluctuations. The surface and calcic horizons of an Olton soil (fine, mixed, superactive, thermic Aridic Paleustoll) were packed into triplicate, 0.5‐m‐tall, 100‐L columns and wetted. We compared θv determined by volumetric measurements, time domain reflectometry (TDR), and MCAPs. The TDR θv were within ±0.01 m3 m−3 of volumetric determinations for air‐dry and saturated soil. The factory supplied universal MCAP calibration provided accurate θv estimates for air dry (±0.01 m3 m−3) surface and calcic soil materials but not after wetting (≈−0.05 m3 m−3). Also, imprecise MCAP sensor positioning during water frequency parameter determination was problematic and biased initial θv measurements. After calibration against TDR, the MCAP θv varied ±0.01 m3 m−3 from measured θv for air‐dry and saturated conditions for both soil materials, which were then pooled to obtain one calibration. Column resaturation with saline water affected permittivity and elevated MCAP θv ≈0.25 m3 m−3 above the available pore space. Cyclical soil temperature fluctuations of 15°C induced similar fluctuations in indicated θv throughout the column (0.04 m3 m−3 for MCAP and 0.02 m3 m−3 for TDR), which was attributed to variations in permittivity.

When crops are grown in a row configuration, heat and mass transfer within the soil‐canopy system influence the energy and water balance of the crop. Field experiments were conducted near Lubbock, TX, to examine the energy balance of the soil and canopy separately, in cotton (Gossypium hirsutum L.) under a variety of aerial and soil moisture conditions. Bowen ratio techniques were used to obtain the field energy balance, including total latent heat flux (LE). Latent heat flux from the crop canopy (LEc was determined from sap flow measurements of transpiration. Latent heat flux from the soil (LEs) was computed as the daerence between LE and LEc. These measurements were coupled with radiation measurements at the soil surface to partition the energy balance into soil and canopy components every 12 min throughout the day. Results indicate that detailed measurements of energy exchange within the soil‐canopy‐atmosphere system can be obtained without making simplifying assumptions about energy transfer. Daily energy balances were strongly influenced by sensible heat transport, and the radiation balance alone did not account for the magnitude or diurnal pattern of LES and LEC. When the soil surface was dry, the canopy simultaneously absorbed sensible heat originating from the soil and above‐canopy air, accounting for more than 21 and 12% of LEC respectively. After an irrigation, LES accounted for more than 50% of LES even when the leaf area index was greater than two, and 11 to 21% of daily LEC occurred at night. The soil surface absorbed sensible heat from the canopy after irrigation, which increased LE1 while decreasing LEC. Analysis indicates that within‐canopy radiative and convective energy transfer must be considered to accurately characterize LES and LEC in row crops during periods of partial cover.

Dryland crops grown in semiarid environments often do not completely cover the soil, leaving a portion of the soil surface exposed to a condition of rapid soil‐water evaporation. Quantitative separation of soil evaporation and crop transpiration is important if cultural practices or cultivars are to be evaluated. This study was designed to evaluate a combined energy and water balance model, ENWATBAL, to describe the concurrent heat and water fluxes in a row crop. Inputs to the model include soil and plant variables and daily weather data. Measurements were made for a period of 74 d over a cotton (Gossypium hirsutum L.) canopy during 1985 on an Olton soil (fine, mixed, thermic Aridic Paleustolls) at Lubbock, TX. Data collected included soil‐water content, soil temperature, root distribution, soil evaporation with microlysimeters, and leaf area index, for both an irrigated and a dryland plot. The values for daily evaporation and evapotranspiration calculated with the model were within 1 standard deviation of the measured values. Cumulative evaporation and evapotranspiration from the model agreed with measured values within 7% for the dryland and 8% for the irrigated plot. Estimated soil‐water and temperature profiles also agreed closely to measured values. Soil evaporation was found to be 30% of evapotranspiration, for both the irrigated and the dryland plot. The ENWATBAL model provides a reliable method of evaluating the effects of management practices and crop selection on the water‐use efficiency of crop production in a semiarid area.

system with winter wheat (Triticum aestivum L.), corn (Zea mays L.), and grain sorghum [Sorghum bicolor There is a need for an accurate method to calculate and to measure (L.) Moench] as the main crops; whereas, in the southcrop water use on real-time. We implemented a system that combines knowledge of crop water use and available technology to control the ern region cotton is the principal crop. timely application of water. Our objective was to test the system and Water management for irrigation of cotton and other compare it to the empirical engineering approach that uses a crop crops in the southern THP should consider three factors: coefficient to relate crop water use to a reference evapotranspiration. (i) rain distribution and amount; (ii) well-capacity and Technologies involved are the measurement of plant water use with application system used, for example, furrow, sprinkler, stem flow gauges, of soil water with time domain reflectometry, and low energy precision application (LEPA) (Lyle and weather variables. Measurements are coupled with calculated values Bordovsky, 1981), and drip; and (iii) scheduling acof crop water use obtained with the model ENWATBAL. A single cording to the soil water balance, and the crop's needs computer controls all functions, for example, measurements, model and physiological response to water. Rain distribution execution, activation of water delivery system. The system was tested and amount is important before, during, and after the for a 2-yr period with cotton (Gossypium hirsutum L.) in Lubbock, TX, using surface drip irrigation. Field experiments were conducted growing season. For example, late fall and early spring on an Olton clay loam (fine, mixed, superactive, thermic Aridic Paleus-rains can adequately fill the soil profile and provide the tolls). Comparison of measured and calculated values of crop transpicotton crop with adequate water during the growing ration and soil water evaporation were in close agreement. Simulated Abbreviations: DOY, day of year; ENWATBAL, energy and water R.J. Lascano, Texas A&M Univ. Res. and Ext. Center, Rt. 3, Box balance model; LAI, leaf area index (m 2 m Ϫ2 ); LEPA, low energy 219, Lubbock, TX 79403-9757 and USDA-ARS, 3810 4th St.).where LE d is the daily latent heat flux (in W m Ϫ2 ) (d refers to daily totals), D e is the desorptivity (in W m Ϫ2 d 1/2 ), t is the Published in Agron. J. 92:832-836 (2000).