R. J. Lascano

THE SPATIAL SENSITIVITY OF TIME-DOMAIN REFLECTOMETRY

With any method for measuring soil water content it is useful to know the spatial sensitivity of the measurement, i.e., what volume of soil is measured and what the distribution of sensitivity is within that volume. We determined this information for the method of time-domain reflectometry (TDR), using a laboratory procedure in which the distribution of water surrounding the waveguides was controlled to give different spatial arrangements of water around the waveguides. The sensitivity is largely confined to a region with a cross-sectional area of approximately 1000 mm2 surrounding the waveguides, although a limited sensitivity extends much farther, enclosing 3500 to 4000 mm2. The width of the region of sensitivity normal to the plane containing the wave-guides is approximately 30 mm, indicating that the method should allow excellent depth resolution when the waveguides are installed horizontally, and that it should be possible to place such waveguides within 20 mm of the surface. Finally, there was no discernible variation in sensitivity longitudinally (along the length of the wave-guides).

Soil and canopy energy balances in a west Texas vineyard

Water use in vineyards is controlled by energy absorbed by plants and the soil surface. An 8 day field experiment was conducted in a commercial vineyard near Lamesa, TX, to evaluate soil and canopy energy balances, and to examine energy exchange between canopy and soil. Grape- vines in the vineyard were wrapped tightly to trellis wires, creating compact hedgerows that were 3 m apart and of 1.6 m height and 0.4 m width, with little foliage below 1 m above the soil surface. The Bowen ratio method was used to measure the vineyard energy balance including total latent heat flux (AE). Latent heat flux from the canopy (,~Ec) was determined from sap flow measurements of transpiration. Soil latent heat flux (AEs) was calculated as the difference between AE and AE c. These measurements were combined with measurements of soil net irradiance to partition the vineyard energy balance into soil and canopy components. During the study, AEs accounted for 44-68% of AE. Unstable conditions predominated during the study, with the soil generating sensible heat that was transferred to the canopy, producing values of AE c that were greater than canopy net irradiance. Within-row advection of sensible heat was 17-36% of AEc. Although the canopy was cooler than within- and above-canopy air, it was not a strong enough sink for sensible heat to produce stable conditions above the canopy. The narrow hedgerows created an unusual diurnal pattern of canopy net irradiance, having midmorning and midafternoon peaks, and a low midday plateau. Morning and afternoon peaks occurred during times of maximum direct beam irradiance on east and west sides of the hedgerows. Results also showed that within-canopy wind speed and air temperature were affected by wind direction.

Determination of soil water evaporation and transpiration from energy balance and stem flow measurements

Frequent measurements of soil water evaporation (E) and transpiration (T) are needed to quantify energy and water balances of sparse crops. Field experiments were conducted in Lubbock, TX to examine the feasibility of partitioning evapotranspiration (ET) from a cotton crop (Gossypium hirsutum L.) during periods of partial cover. The Bowen ratio energy balance method and heat balance stem flow measurements were used to make near-instantaneous measurements of ET and T, respectively. Transpiration on a unit land area basis was determined by normalizing stem flow measurements by leaf area or plant density. Soil water evaporation was computed as the difference between ET and T. The accuracy of the method was evaluated by comparing calculated values of E with measured values obtained from soil microlysimeters. Measurements over an 8-day period following an irrigation indicated that daily values of calculated E were within 0.5 mm of measured values in six out of seven comparisons when stem flow measurements were normalized on a leaf area basis. On average, daily calculated E was within ±11% of measured values. Calculated and measured cumulative E agreed to within 0.6 mm at the end of the evaluation period. Computing T by normalizing stem flow on a plant density basis resulted in overestimates of T and underestimates of E. Error analysis indicates that the precision of the E estimate decreases rapidly as evaporation becomes a smaller fraction of ET, and is influenced equally by the resolution of the stem flow and leaf area measurements. This study demonstrates that high frequency, independent measurements of soil and canopy evaporation can be obtained by measurement of ET and stem flow.

Effects of trellising on the energy balance of a vineyard

Abstract Field experiments were conducted in 1992 and 1993 in a commercial vineyard near Lamesa, TX, to evaluate soil and canopy energy balances. In 1992, grapevines were wrapped tightly to trellis wires, creating compact hedgerows that were 3 m apart, 1.6 m high and 0.4 m wide with little foliage below 1 m above the soil surface. In 1993, vines were allowed to grow outward and downward from the trellis because of concerns that excess shading of vines and fruit had occurred the previous year. This change in trellising created wider, less dense hedgerows that increased sunlit leaf area and reduced sunlit soil area from the previous year. Leaf area was also 55% larger in 1993. We examined how the change in trellising affected soil and canopy energy balances. The Bowen ratio method was used to measure the vineyard energy balance including total latent heat flux (λE). Latent heat flux from the canopy (λEc) was determined from sap flow measurements of transpiration. Soil latent heat flux (λEs) was calculated as the difference between λE and λEc. These values were combined with measurements of soil net irradiance to partition the vineyard energy balance into soil and canopy components. The change in trellising in 1993 had little effect on vineyard net irradiance (Rn) and λE, but did alter the partitioning of Rn and λE into soil and canopy components. Canopy Rn and λE were substantially higher for the open hedgerows in 1993 whereas soil Rn and λE were lower than for the dense hedgerows in 1992. Both trellising and leaf area contributed to changes in the energy balance. A comparison of λEc per unit land area with λEc per unit leaf area suggested that roughly 60% of the difference in λEc between years was caused by the change in trellising.

Cotton Irrigation Management with LEPA Systems

Irrigations were applied to short-season cotton at Halfway, Texas, using a LEPA irrigation system at intervals of 2, 4, 8, and 12 days in 1986 and at 3, 6, 9, and 18 days in 1987 and 1988. The four interval treatments were grouped for analysis and referred to as 3D, 5D, 9D, and 15D. Irrigation quantities at each interval were 0.4, 0.6, 0.8, and 1.0 times a base irrigation amount (BI) which equalled estimated cotton evapotranspiration (ET) less rainfall. The DRY treatments received preplant irrigations only.

Laboratory Evaluation of a Commercial Dielectric Soil Water Sensor

Development of management strategies for efficient water utilization of crop production requires measurements of changes in soil water content on a dynamic basis. Many of the methods currently used for measuring these changes are destructive, slow, or relatively expensive for large-scale investigations. A commercially available, low-cost, nondestructive soil moisture sensor for measuring changes in soil volumetric water content (VWC) on the basis of changes in the dielectric constant of the soil water was evaluated under laboratory conditions for two soil series (Amarillo fine sandy loam [fine-loamy, mixed, superactive, thermic Aridic Paleustalfs] and Pullman clay loam [fine, mixed, thermic Torretic Paleustolls]) and a potting material across a wide range of water contents. Probes were placed in containers filled with deionized water and soil. Containers with Amarillo fine sandy loam were placed in a programmable temperature chamber and subjected to a series of changes in both temperature and VWC. Containers with Pullman soil and potting material were only subjected to changes in VWC at a constant temperature. Probe output at a constant temperature between air dry and a VWC of 0.25 m 3 m −3 was linear for the Pullman soil and potting material and nonlinear for the Amarillo soil. When the Amarillo soil temperature varied between 15.9 and 39.1°C −1 at a constant VWC, probe output changed the equivalent of 0.10 m 3 m −3 . The temperature sensitivity was 0.5 mV °C −1 for air-dry and about 5 mV °C −1 for wet Amarillo soil. We conclude that probe output is soil specific and, given the nonlinear response to increasing water content on some soils and sensitivity to temperature, will require soil-specific calibration equations.

Effects of crop residue on soil and plant water evaporation in a dryland cotton system

SummaryDryland agricultural cropping systems emphasize sustaining crop yields with limited use of fertilizer while conserving both rain water and the soil. Conservation of these resources may be achieved with management systems that retain residues at the soil surface simultaneously modifying both its energy and water balance. A conservation practice used with cotton grown on erodible soils of the Texas High Plains is to plant cotton into chemically terminated wheat residues. In this study, the partitioning of daily and seasonal evapotranspiration (Et) into soil and plant water evaporation was compared for a conventional and a terminated-wheat cotton crop using the numerical model ENWATBAL. The model was configured to account for the effects of residue on the radiative fluxes and by introducing an additional resistance to latent and sensible heat fluxes derived from measurements of wind speed and vapor conductance from a soil covered with wheat-stubble. Our results showed that seasonalEt was similar in both systems and that cumulative soil water evaporation was 50% ofEt in conventional cotton and 31% ofEt in the wheat-stubble cotton. Calculated values ofEt were in agreement with measured values. The main benefit of the wheat residues was to suppress soil water evaporation by intercepting irradiance early in the growing season when the crop leaf area index (LAI) was low. In semiarid regions LAI of dryland cotton seldom exceeds 2 and residues can improve water conservation. Measured soil temperatures showed that early in the season residues reduced temperature at 0.1 m depth by as much as 5°C and that differences between systems diminished with depth and over time. Residues increased lint yield per unit ofEt while not modifying seasonalEt and reducing cumulative soil water evaporation.

Spatial and Temporal Variability of Sorghum Grain Yield: Influence of Soil, Water, Pests, and Diseases Relationships

This study was conducted to determine relationships between biotic and abiotic factors and to generate information needed to improve the management of site-specific farming (SSF). The effects of water (80% evapotranspiration (ET) and 50% ET), hybrid (drought-tolerant and -susceptible), elevation, soil texture, soil NO3--N, soil pH, and greenbugs (Schizaphis graminum) (Gb) on sorghum grain yield were investigated at Halfway, TX on geo-referenced locations on a 30-m grid in 1997, 1998, and 1999. Grain yields were influenced by interrelationships among many factors. Grain yields were consistently high under 80% ET treatment and in the upper slopes where the clay and silt fractions of the soil were high. Soil NO3--N, rainfall, hybrid, and Gb effects on grain yields were seasonally unstable. Soil NO3--N increased grain yield when water was abundant and depressed grain yields when water was limiting. Plant density effects on grain yield were confounded with hybrid responses to drought and Gb infestation. Managing seasonally unstable factors is a major challenge for farmers and better ways to monitor crop growth and diagnose causes of poor plant growth are needed. To improve the management of SSF, effects of the relationships between biotic and abiotic factors on crop yield must be integrated and evaluated as a system. Based on our study, information on seasonally stable factors like elevation and soil texture is useful in identifying management zones for water and fertilizer application. Water and fertilizers management should be complemented by in-season management of seasonally unstable factors like soil NO3--N, rainfall, hybrid, and Gb effects on grain yield.

Spatial and Temporal Variability of Corn Grain Yield: Site-Specific Relationships of Biotic and Abiotic Factors

Inadequate information on factors affecting crop yield variability has contributed to the slow adoption of site-specific farming (SSF). This study was conducted to determine the effects of biotic and abiotic factors on the spatial and temporal variability of irrigated corn grain yields and to derive information useful for SSF. The effects of water (80% evapotranspiration (ET) and 50% ET), hybrid (drought-tolerant and -susceptible), elevation, soil index (SI)(texture), soil NO3–N, arthropods, and diseases on corn grain yield were investigated at Halfway, TX on geo-referenced locations. Grain yields were influenced by interrelationships among biotic and abiotic factors. Grain yields were consistently high under high water treatment, at higher elevations, and on soils with high SI (high clay and silt). Soil NO3–N increased grain yields when water was adequate. Management zones for variable rate fertilizer and water application should, therefore, be based on information on elevation, SI, and soil NO3–N. The effects of arthropods, diseases, and crop stress (due to drought and N) on corn grain yield were unpredictable. Spider mite (Oligonychus pratensis) and common smut (Ustilago zeae) damage occurred under hot and dry conditions in 1998. Spider mite infestations were high in areas with high soil NO3–N. Moderate air temperatures and high relative humidity in 1999 favored southwestern corn borer (Diatraea grandiosella) and common rust (Puccinia maydis) incidences. Knowledge of conditions that favor arthropods and diseases outbreak and crop stress can improve the efficiency of scouting and in-season management of SSF. Management of SSF can be improved when effects of biotic and abiotic factors on grain yield are integrated and evaluated as a system.

Test and analysis of a model of water use by sorghum

We carried out a field experiment with a well-developed sorghum crop to validate a water use and uptake model. The calculated evapotranspiration over a 50-d period was not significantly different from the experimental values. Soil water content profiles calculated with the model showed some systematic deviation from the measured values, possibly because the physical nature of the profile was assumed to be homogeneous. The spread of the simulated results reflects the field variation of hydraulic soil properties.-