Average Daily Evapotranspiration Research Articles

Daily potential evapotranspiration and diurnal climate forcings: influence on the numerical modelling of soil water dynamics and evapotranspiration

A physically based, variably saturated flow model was developed to predict soil water dynamics, evapotranspiration (ET) from the vadose zone, and recharge to (or exfiltration from) the saturated zone using mean daily atmospheric forcings and to identify the value of diurnal climate forcings on those predictions. The vadose zone flow is modelled using the Galerkin finite element technique to solve Richards’ equation in one-dimension. The model was able to accurately predict measured soil moisture, water table elevation and actual ET at Paynes Prairie State Preserve in Florida. The forecast Nash–Sutcliffe efficiencies of actual ET, water table and average soil moisture content increased modestly, from 0.605–0.653, 0.888–0.916 to 0.902–0.913, respectively, when the average daily ET forcing was replaced with a diurnal evaporation cycle. Several additional numerical experiments were conducted to evaluate the influence of the evaporation cycle disaggregation approach on modelled ET and soil moisture content for different soil textures, vegetation surfaces, and water table depth. The results show that the enhanced predictive value of the diurnal ET cycle increases with decreasing vegetation, decreasing clay content, and increasing water table depth. Using numerical studies, actual evaporation is shown to be higher for daily average evaporation as compared to the diurnal cycle evaporation for specific ranges of shallow water table depth. For clay soils, this range occurs from approximately 40 to 300 cm below land surface for bare soils and from approximately 40 to well below 300 cm below land surface for vegetated soils. The range for sandy soils is approximately 80–200 cm below land surface for both bare and vegetated soils. Within this range, the maximum difference of the actual to potential evapotranspiration ratio for the clay soil, resulting from using different forcing methods, is 20 and 10% for bare soil and vegetated conditions, respectively. The forcing method choice is more important for sandy soils with ratio differences of 50 and 16% for bare soil and vegetated conditions, respectively.

Determining water consumption in olive orchards using the water balance approach

Efficient irrigation regimes are becoming increasingly important in commercial orchards. Accurate measurements of the components of the water balance equation in olive orchards are required for optimising water management and for validating models related to the water balance in orchards and to crop water consumption. The aim of this work was to determine the components of the water balance in an olive orchard with mature ‘Manzanilla’ olive trees under three water treatments: treatment I, trees irrigated daily to supply crop water demand; treatment D, trees irrigated three times during the dry season, receiving a total of about 30% of the irrigation amount in treatment I; and treatment R, rainfed trees. The relationships between soil water content and soil hydraulic conductivity and between soil water content and soil matric potential were determined at different depths in situ at different locations in the orchard in order to estimate the rate of water lost by drainage. The average size and shape of the wet bulb under the dripper was simulated using the Philip’s theory. The results were validated for a 3 l h −1 dripper in the orchard. The water amounts supplied to the I trees during the irrigation seasons of 1997 and 1998 were calculated based on the actual rainfall, the potential evapotranspiration in the area and the reduction coefficients determined previously for the particular orchard conditions. The calculated irrigation needs were 418 mm in 1997 and 389 mm in 1998. With these water supplies, the values of soil water content in the wet bulbs remained constant during the two dry seasons. The water losses by drainage estimated for the irrigation periods of 1997 and 1998 were 61 and 51 mm, respectively. These low values of water loss indicate that the irrigation amounts applied were adequate. For the hydrological year 1997–1998, the crop evapotranspiration was 653 mm in treatment I, 405 mm in treatment D and 378 mm in treatment R. Water losses by drainage were 119 mm in treatment I, 81 mm in treatment D and 4 mm in treatment R. The estimated water runoff was 345 mm in treatments I and R, and 348 mm in treatment D. These high values were due to heavy rainfall recorded in winter. The total rainfall during the hydrological year was 730 mm, about 1.4 times the average in the area. The simulated dimensions of the wet bulb given by the model based on the Philip’s theory showed a good agreement with the values measured. In a period in which the reference evapotranspiration was 7.9 mm per day, estimations of tree transpiration from sap flow measurements, and of evaporation from the soil surface from a relationship obtained for the orchard conditions, yielded an average daily evapotranspiration of 70 l for one I tree, and 48 l for one R tree.

Carbon dioxide fluxes over bermudagrass, native prairie, and sorghum

The Bowen ratio/energy balance (BREB) method was used to measure 30 min water vapor and carbon dioxide (CO 2) fluxes over three fields dominated by different C 4 grasses (bermudagrass, tallgrass native prairie, and sorghum) at the Blackland Research Center, Temple, TX. Fluxes were related to biotic and abiotic phenomena. Carbon accumulation rates calculated from BREB measurements were compared with those determined from plant biomass measurements. Thirty min CO 2 fluxes were measured continuously from 22 February to 22 November (days 53–326) in 1993 and from 3 March to 22 November (days 62–326) in 1994. Soil CO 2 fluxes were measured periodically in each field and from a bare soil. In all fields, average daily evapotranspiration varied from less than 1 mm day −1 in March and November to about 5 mm day −1 in June and July. The CO 2 fluxes, which were affected by leaf area, radiation, and soil water content, were near zero in each field in the fall and spring and were maximum in the summer (positive toward surface). Total annual CO 2 flux in the bermudagrass, which was sprigged (planted with grass stolons) in April 1993, was −0.4 and 2.8 kg of CO 2 m −2 year −1 (which are equivalent to −0.1 and 0.8 kg of carbon (C) m −2 year −1) in 1993 and 1994, respectively. The larger flux in the second year was due to greater bermudagrass leaf area. Corresponding annual CO 2 fluxes were 0.2 and 0.3 kg m −2 year −1 (0.05 and 0.08 kg C m −2 year −1) for the prairie, and 0.9 and −0.3 kg m −2 year −1 (0.2 and −0.09 kg C m −2 year −1) for the sorghum. Bare soil CO 2 flux was a small fraction of soil CO 2 flux from all fields, suggesting that a considerable amount of soil CO 2 flux was due to root respiration. The dry matter accumulation rate calculated for each year and field from CO 2 flux measurements was within 20% of the rate calculated from gravimetric biomass measurements. The prairie and sorghum fields were in approximate equilibrium for C storage because estimated annual CO 2 fluxes were near zero. By contrast, the bermudagrass field was a large C sink, primarily in the roots, during the second year. This substantiates other evidence that conversion from continuously cultivated cropland to grassland could create a short-term C sink.

Effect of removal of Juniperus ashei on evapotranspiration and runoff in the Seco Creek Watershed

The water balance of a watershed may be affected by replacing deep‐rooted woody species with shallow‐rooted herbaceous vegetation. The objective of this study was to measure the effect of removing an individual species of tree, Juniperus ashei (Bucch.), on the runoff (RO) and evapotranspiration (ET) from two adjacent, unreplicated 15‐ha areas (termed untreated and treated) in northeast Uvalde County, Texas, U.S.A. Daily ET from the two areas, measured from 1991 through 1995 using the Bowen ratio–energy balance method, varied from near 0 to 6 mm/d. All J. ashei taller than 0.5 m were cut with a chain saw in the treated area in September 1992. During both the pretreatment period (prior to September 1992) and the posttreatment period, the slope of treated ET as a function of untreated ET was ∼1, suggesting that for the entire period of measurements, brush removal had no significant effect on ET. Average daily ET from the area to be treated was 0.05 mm/d lower than that from the untreated area during the 2‐year pretreatment period, while it was 0.12 mm/d lower during the 3‐year posttreatment period. The ET difference (untreated minus treated) was 0.3 mm/d in the first 2 years following removal of J. ashei and decreased thereafter. Removal of J. ashei had no consistent effect on RO. Vegetation management increased the potential for greater water yields in the short term from these rangelands by decreasing ET for the first 2 years after imposition of treatment.

Evapotranspiration of herbaceous mimosa (Mimosa strigillosa), a new drought-resistant species in the southeastern United States

Abstract Herbaceous mimosa (Mimosa strigillosa Torr and Gray) is a new reclamation species currently under a series of assembly and evaluation programs by the Natural Resources Conservation Service's Plant Materials Center and Stephen F. Austin State University at Nacogdoches, Texas. Two floating lysimeters were designed and installed at the Stephen F. Austin Experimental Forest, about 20 km south of Nacogdoches, Texas to study differences in evapotranspiration (ET) between the mimosa and bare ground in July–October 1993. Pan coefficients during the study period ranged from 0.40 to 0.58, and no irrigation was applied to the lysimeters in the study. Water losses to the air in area covered by herbaceous mimosa were compatible with other turfgrasses under optimum soil moisture conditions. However, under environmental stress, average daily ET for the mimosa in four summer months was 1.6 mm/day, a value only 0.4 mm/day greater than bare ground. These results agree with a previous study that the plant is a water conservation species capable of tolerating severe drought conditions.

Errors in Penman-Wright Alfalfa Reference Evapotranspiration Estimates: II. Effects of Weather Sensor Measurement Variability

The effects of weather measurement variability on Penman-Wright alfalfa reference evapotranspiration (ETr) estimates were investigated. Measurement variability or error was estimated in two ways(1) using manufacturers specifications of accuracy, and (2) by actual measurement. Results showed that evapotranspiration (ET) errors due to combined random and systematic error in weather measurements were less than 5.5% of average daily ETr when sensors were operating within error level envelopes equivalent to their specifications of accuracy. When sensors were operating at the limits of their specifications of accuracy, the greatest ET error was due to solar radiation measurement error, followed by dewpoint, maximum temperature, and wind speed measurement errors. Weather measurement variability derived from sensor evaluations resulted in different ET error. Air temperature measurement variability was small yielding ET errors less than 4% of average daily ET. Dewpoint temperature errors, based on relative humidity measurement error of 10%, resulted in ET errors of about 6 to 7% of average daily ET. Solar radiation measurement variability resulted in ET errors of nearly 8.5% of average daily ET.

Evapotranspiration of upland paddy

The cumulative evapotranspiration for early maturing varieties (duration 85-116 days) of upland paddy was found to be in the range of 430, 5 to 598.9 mm. The average daily evapotranspiration over the crop period was found to be 4,67 to 6.07 mm/day depending upon varietal type and location. The booting, heading and flowering stages as critical stages as these exhibit peak period of water consumption. The water use efficiency of the crop varied from 2.31 to 8, 34 kg/ha/mm.

Water use and yield relationships of irrigated potato

The experiment was conducted at Joydebpur, Bangladesh, during the rabi (dry) seasons of 1986–1987 and 1987–1988 with potato cultivar “Cardinal”. Four treatments: no irrigation, irrigation at 80%, 60% and 40% depletion of available soil moisture ( asm) were tested. For both the years five irrigations given at 40% depletion of asm gave maximum yield (22.3 t ha −1) which was significantly different from the other treatments. On average 40 mm water was applied at an interval of around 12 days starting from 30 days after planting. The same treatments resulted in maximum number of stems per hill, highest foliage coverage and maximum number of compound leaves; maximum weight and number of tubers as percentage of total yield in the range of 28–45 mm and > 45 mm in size. It also resulted in maximum overall water use efficiency of 81.0 kg ha −1 mm −1, irrigation water use efficiency of 79.5 kg ha −1 mm −1 and a maximum total water use of 267 mm. The average daily evapotranspiration varied from 1.0–5.1 mm. The non-irrigated treatment gave the highest percentage of tubers smaller then 28 mm.

Evapotranspiration, Yield, and Water-Table Studies of Celery

ABSTRACT STUDIES of the evapotranspiration (ET) and water use efficiency for celery in relation to water-table depth were conducted in nine lysimeters on organic soil. The water tables were controlled at 30, 60, and 85 cm with three replications. Two crops of celery were raised. The celery ET and water use efficiency are inversely related to the water-table depth. The average daily ET varied from 3.0 to 4.9 mm/day. The celery required about 23 to 34 kg of water to produce 1 kg of fresh biomass, and required about 33 to 50 kg of water to produce 1 kg of marketable yield. The ratio of ET to the standard pan evaporation increased steadily, about 11% per week, throughout the entire growing period. The celery grown over the 30 cm water table was as good yield as over the water tables of 60 and 85 cm

Evapotranspiration of Lettuce in Relation to Water Table Depth

ABSTRACT STUDIES of the evapotranspiration (ET) and water use efficiency for lettuce in relation to water table depth were conducted in nine lysimeters on organic soil. The water tables were controlled at 45, 60, and 85 cm with three replications. Two crops of lettuce were raised. The lettuce ET and water use efficiency are inversely related to the water table depth. The average daily ET varied from 2.0 to 3.4 mm/day. The lettuce required about 14 to 24 kg of water to produce 1 kg of fresh biomass, and required about 19 to 35 kg of water to produce 1 kg of marketable yield. The ratio of ET to the standard pan evaporation increased steadily to a peak around the lettuce heading development period, then the ratio declined toward the harvest. The lettuce grown in the 60 cm water table is as good as the conventional water table (85 cm).

Water requirements for mesquite (Prosopis juliflora)

Evapotranspiration (ET) determined by different investigators, compared with values determined by the White and Troxell methods, showed that their values provided reasonable estimates and that utilizing diurnal water table fluctuations furnishes a method of computing ET with less than 100% vegetation density. Average daily maximum ET rates for June were calculated, and ratios were compared with values from other independent studies. Since ET rates, determined by White and Troxell methods, indicated plausible values of water use when compared to other independent studies, they could be considered index values useful as guidelines for water abstracted by mesquite.