Predict Water Use Research Articles

A SVAT scheme describing energy and CO 2 fluxes for multi-component vegetation: calibration and test for a Sahelian savannah

A soil vegetation atmosphere transfer scheme (SVAT), describing the fluxes of heat, water vapour and CO 2 between a multi-component vegetated land surface and the atmosphere, has been developed. The SVAT has been calibrated and tested using 30 days of micrometeorological and physiological measurements collected over a sparse savannah in the Sahel. The model explicitly represents the four major functional components of the savannah: shrubs (C 3 photosynthesis), grasses (C 4), herbs (C 3) and bare soil. The leaf conductance model for the shrub, grass and herb components was calibrated against porometry measurements made in the field. The performance of the SVAT was tested against independent surface flux and temperature measurements. Agreement between the model predictions and measurements of the total net radiation, sensible and latent heat fluxes, net CO 2 exchange and surface temperatures was good: in all cases at least 80% of the variance in the measurements was explained by the model. Separate flux and surface temperature predictions for the four surface components were satisfactory, although complete verification was difficult as data were lacking for some variables. Predicted water use efficiency (WUE) of the vegetation showed a strong, non-linear dependence on vapour pressure deficit, especially for the bushes and the grasses. WUE for the grasses was about three times as large as the values found for the bushes and herbs. A SVAT model like this can be employed to address the possible effects of CO 2 enrichment and climate change on the competitive balance between different species in a plant community or in research concerning productivity and water use efficiency in mixed crops, such as agroforestry systems.

Water Use Efficiency as a Method for Predicting Water Use by Weeds

Water use by weeds is one type of loss that contributes to the cost of weeds to agriculture. Water use efficiency provides a mechanism by which water use by weeds can be estimated. Uncontrolled weed growth could result in estimated irrigation costs exceeding $50.00/ha. Weeds like barnyardgrass at threshold densities in irrigated crops like sugarbeet or tomato are estimated to increase production costs by about $20.00/ha depending on water cost.

Relationships Between Water Availability and Selected Vessel Characteristics in Eucalyptus Grandis and Two Hybrids

The primary objective of this study was to determine the relationships between water availability, plant growth and selected vessel characteristics for Eucalyptus grandis and two hybrids, so as to ascertain whether these xylem characteristics predict water use efficiency. Cuttings of Eucalyptus grandis, E. grandis × camaldulensis and E. grandis × nitens were planted in 220 litre drums from which rainfall was excluded. One half of the individuals received a low watering treatment; one half received a higher watering treatment. Soil moisture depletion through root uptake was monitored weekly and the removed water replaced to maintain 60 and 80 litres in the pots of the low and high watering treatments respectively. Mean values for tangential vessel diameter, vessel frequency and vessel element length were compared for the two treatments. In E. grandis and the hybrid E. grandis × camaldulensis vessel diameter (P < 0.01 ' P < 0.05 respectively) and vessel element length (P < 0.05 for both) increased from the dry to the wet treatment as water uptake through transpiration increased. There is no significant correlation between available water and vessel frequency. For E. grandis × nitens, on the other hand, only vessel frequency was significantly (P < 0.01) correlated with water uptake. In all three species/hybrids water availability also had a significant influence on stem diameter (P < 0.0001) and transverse sectional stem area (P < 0.0001) which increased with increased water consumption. These results suggest that E. grandis × nitens may be more water use efficient than E. grandis, which is commonly grown for timber and thus could potentially be used as a replacement species that is more water conservative in this water limited region.

Computer model for managing saline water for irrigation and crop growth: preliminary testing with lysimeter data

A computer model was developed to help formulating irrigation strategies for areas where use of saline waters of questionable quality may be an option for crop production. The model estimates salt and water balance in the soil profile and predicts crop growth in response to weather and irrigation and crop rotation management. Salinity effects on crop growth are simulated in the model through inclusion of osmotic potential in the determination of water uptake. However, it was found that osmotic potential alone did not explain reduced crop growth under heavy and frequent irrigation. Salinity effects on root permeability were found to satisfactorily explain reduced water uptake and growth under such conditions. The model was calibrated and tested for wheat and sorghum crops using one year's data from column lysimeters irrigated with water of varying qualities at the U.S. Salinity Laboratory at Riverside, California. The calibrated model was found to reasonably predict water use, soil salinity, and crop production of independent test lysimeters. Although the model appears to have potential for analyzing irrigation strategies in agricultural systems with salinity constraints, additional testing with field data will be required.

Evapotranspiration Measurement and Modeling for a Potted Chrysanthemum Crop

A crop of chrysanthemums was grown on a lysimeter in a variable shade greenhouse. Measurements were made of evapotranspiration (ET), air velocity, air temperature, air dew point temperature, carbon dioxide concentration, leaf temperature, leaf area, and photosynthetically active and global solar radiation. Comparisons were made between the evapotranspiration of the crop and the predicted water use from a computer model that used temperature, relative humidity, and solar irradiance levels as inputs (the combination model). The ET was studied as a function of vapor pressure deficit (VPD) and solar irradiance levels both separately and together. A relationship between stomatal resistance and solar irradiance levels for chrysanthemums was established.

Responses of Tea (Camellia sinensis) to Irrigation and Fertilizer. II. water Use

SummaryThe water use of clonal tea (Clone 6/8) in a line-source irrigation × nitrogen experiment in the Southern Highlands of Tanzania was monitored with a neutron probe during 1987 and 1988 to a depth of 3 m and 1989 to a depth of 5 m. The results for 1989 were used to calibrate a single layer water balance model which was then used to estimate water use for the preceding 1986, 1987 and 1988 dry seasons. In the model evapotranspiration was reduced linearly when the soil water deficit exceeded a critical value of 60 mm. The model predicted water use well for unirrigated, partially irrigated and fully irrigated treatments. Total extractable water was 330 to 350 mm in the 5.5 m deep root zone and estimated annual water use in 1988/89 ranged from 800 mm for unirrigated to 1200 mm for fully irrigated plots, of which about 400 mm was used in the wet season from December to April. Water use efficiencies were between 1 to 4 kg of made tea ha−1 mm−1, increasing with improving water and nutrient status. The slope (b) of the relation between the relative yield loss and the relative reduction in water use was very steep (b = 1.3) reflecting the sensitivity of shoot growth and yield of this clone to water stress.

Evapotranspiration Model for Developing Crops

ABSTRACT Crop water use is a complex function of climatic conditions, stage of crop development and soil water content. Using principles of heat and mass balance, a comprehensive evapotranspiration model was developed to predict water use, soil evaporation, transpiration and microclimate of a crop where the soil may go through cycles ofwetting and drying. This model accounts for the sensible and latent energy exchanges and feedback effects within the canopy air space, and among the canopy, soil, and ambient air above the canopy. Using an electrical analog, equations for energy fluxes in the soil-plant-atmosphere continuum were developed. Solutions were developed that allow practical applications and evaluation of the model using measured weather, plant, and soil data as inputs. The model describes the influence of soil evaporation on plant transpiration and the changes in plant temperature under changing vapor pressure deficit within the canopy. Bragg soybeans were grown in 330, 660, and 990 ppm CO2 concentrations under well irrigated conditions in outdoor growth chambers to obtain a data base to evaluate the ability of the model to predict water use during canopy development. The model accurately predicted diurnal water use patterns as well as the reduction in canopy ET under the higher CO2 levels. Average daily differences between measured and simulated evapotranspiration were less than 4% and these differences were not significant at the 95% level of confidence..

Wheat canopy temperature: A practical tool for evaluating water requirements

Canopy temperatures were measured on durum wheat grown in six differentially irrigated plots. Soil water content was measured by using a neutron‐scattering technique at two locations within each plot. Water contents, in 20‐cm increments to 160 cm, were determined two to five times per week. Using a sliding cubic smoothing technique, we calculated daily water contents and thus water depletion rates for the entire growing season. Canopy temperatures were measured daily between 1330 and 1400 hours. Air temperatures measured at 150 cm above the soil surface were subtracted from the canopy temperatures to form the difference Tc – Ta. The summation of Tc – Ta over time yielded a factor termed the ‘stress degree day’ (SDD). The SDD concept shows promise as an indicator for determining the times and amounts of irrigations. An expression relating evapotranspiration (ET) to net radiation and Tc – Ta was simplified and tested by using ET measurements with a lysimeter. The expression was used to predict water use by wheat in the six plots. Predicted ET and measured water used agreed reasonably well. The expression may be useful in determining amounts of irrigation water to apply.

Further studies of water use by irrigated and unirrigated Arabica coffee in Kenya

1. Frequent soil sampling to 10 ft. depth was carried out on irrigated and unirrigated plots and, with the assistance of rainfall data, fairly accurate measurements of water uso were obtained unless the soil was at, or near to, field capacity. Evapotranspiration could then not bo distinguished from drainage beyond the root range.2. A previous method of relating the water uso of unirrigated coffee to the readings of a sunken evaporation pan was shown to be reliable. The relationships between the water use of unirrigated coffee and meteorological factors (Penman's Eo, sunken pan evaporation or solar radiation) were shown to be dependent on soil-moisturo deficit, and on this basis a more precise method of predicting water use from sunken pan evaporation is proposed. When this method was applied to meteorological data from earlier years, good agreement was obtained between measured and estimated quantities of water in a 10 ft. profile.