Penman Potential Evapotranspiration Research Articles

Potential Evapotranspiration over India - An estimate of Green Water flow

The spatial distribution of Potential Evapotranspiration (PET) is studied by selecting stations that are drawn from the Koppen Climate spectrum of India that spread the length and breadth of the country using the modified Penman potential evapotranspiration equation. Annual and seasonal PET trends are reported. Variation of mean seasonal PET for the country as whole for all the four meteorological seasons and its rate of change are graphically shown. Anomalies in the annual and seasonal PET for the epoch of 1961-1995 with reference to the base period 1931-1960 are brought out. Distribution of climatic types over India according to Koppens climate classification for the period 1951-1980 are presented along with the changes that occurred compared to earlier report. Koppen climate types are characterized in terms of PET to P ratio expressed in percentage. Significant decreasing, increasing and no trend are noticed in annual PET both in space and time. Annual PET for the country as a whole is increased in the latter half of the 20th century. The mean seasonal potential evapotranspiration to precipitation ratio is found to be increased from tropical monsoon climates to dry climate types.

Soil moisture and water use by pastures and silvopastures in a sub-humid temperate climate in New Zealand

Soil moisture content from 0 to 2 m depth was monitored under 2–6 year old radiata pine (Pinus radiata) with three understoreys of bare ground, lucerne (Medicago sativa) and ryegrass/clover (Lolium perenne/Trifolium spp.) and under adjacent open-grown lucerne and ryegrass/clover pastures. By the fifth year soil moisture depletion/recharge pattern under the trees alone was similar to that under open pasture and under trees with pasture understoreys. Maximum plant available moisture storage was 207–223 mm in the top meter of this Templeton silt loam soil but only 69–104 mm at 1–2 m depth where coarse textures often predominated. Lucerne reduced soil moisture content (SMC) to lower levels during drier summers and extracted more water from 1 to 2 m depth than ryegrass/clover. Evapotranspiration (ET) during early summer when soil moisture was high was close to the Penman potential evapotranspiration (Ep), but the difference increased when SMC in the top meter dropped below 200 mm. The silvopasture treatments had higher ET in winter than pasture alone but this was still less than Ep. Soil moisture deficits (SMD) at the end of each summer were sufficiently large to require slightly higher than normal winter rainfall and ET < Ep to recharge the soil to field capacity before the next summer. The soil moisture results, taken together with root and growth data, suggest that trees and understorey pastures are complementary in the first three or four growing seasons but this balance subsequently declines in favor of the pine trees. Management options, to extend the period that understorey pastures are productive, include reducing tree stockings, more vigorous pruning, using competitive understoreys and changing from pines to deciduous trees. Research on new silvopastoral combinations is suggested.

Trends in pan evaporation and reference and actual evapotranspiration across the Tibetan Plateau

The Tibetan Plateau is one of the areas of the world where humans have had a relatively minor impact. The plateau thus provides ideal conditions for investigating evapotranspiration (In this paper, evapotranspiration terms are defined as follows: (1) “Actual evapotranspiration” includes evaporation from water and soil and transpiration from the vegetation of a specific region; (2) “potential evapotranspiration” includes the maximum quantity of water capable of being evaporated from the soil and transpired from the vegetation of a specific surface; (3) “reference evapotranspiration” includes the maximum quantity of water capable of being evaporated from the soil and transpired from a hypothetical reference grass with an assumed height of 0.12 m, a fixed surface resistance of 70 s m−1, and an albedo of 0.23; and (4) “pan evaporation” means evaporation from open circular pans with a diameter of 20 cm.) issues such as temporal trends of evapotranspiration, the pan evaporation paradox, and the complementary relationship hypothesis. We examined Penman‐Monteith reference evapotranspiration and pan evaporation from a 20‐cm pan by using a data set from 75 meteorological observatories across the plateau during the period 1966 to 2003. Actual regional evapotranspiration was estimated in 16 catchments across the plateau during the period 1966 to 2001. Reference evapotranspiration and pan evaporation significantly decreased at 47 and 38% of observatories, respectively, though air temperature at most of sites significantly increased (P &lt; 0.05); wind speed and sunshine hours significantly decreased at 85 and 43% of observatories (P &lt; 0.05). The annual reference evapotranspiration and pan evaporation averaged from all the observatories significantly decreased (P &lt; 0.05) while actual annual evapotranspiration averaged from all the catchments increased (P &lt; 0.1), indicating the existence of a pan evaporation paradox on the plateau. The analysis with a recovered stationary series method showed that decreasing trend in reference evapotranspiration was due to a decrease in wind speed and a decrease in net total radiation, and the increase in air temperature, however, showed little correlation with the declining trends in reference evapotranspiration and pan evaporation. Regional actual evapotranspiration and reference, Penman potential evapotranspiration, or pan evaporation exhibit complementary behavior, which, however, does not support Bouchet’s complementary hypothesis, perhaps because of the very low vapor pressure deficit. The current study suggests that the Bouchet’s complementary relationship needs to be reconsidered at high elevations.

A simple pan‐evaporation model for analysis of climate simulations: Evaluation over Australia

We show that a simple model of pan evaporation (“PenPan”) can be used to analyze monthly mean output from a global climate model (GCM). PenPan is based on a modified version of Penman's potential evapotranspiration equation. Very good agreement is obtained with observed annual pan evaporation for Australian sites when PenPan is forced by surface observations of radiation, wind speed, humidity and air temperature. When PenPan is forced with monthly mean output from the CSIRO GCM, the results are still satisfactory, but pan evaporation is overestimated over southern Australia, primarily due to excessive surface solar radiation simulated by the GCM. The results suggest that PenPan will be a valuable tool for reconciling observed pan‐evaporation trends with climate‐model simulations.

ニホンナシ園における蒸発散速度の実態解明と日蒸発散量の推定

In order to estimate the amount of soil water content of orchards in a region and to improve water management, observations of transpiration and evapotranspiration were made in a Japanese pear orchard. The observations were made over a perild of 4 years, from April to October of each year, using the energy budget (EB), soil water (SW), eddy correlation (EC), and trunk energy budget (TEB) methods. The measured daily transpiration or evapotranspiration was highly proportional to Penman potential evapotranspiration (PET) for almost any month and year. When the inter-year variation was neglected, the coefficient of this proportionality for evapotranspiration (CET) in each month ranged from 0.77 to 0.85. Thus, (0.80 × PET) was proposed as the simplest estimation for daily evapotranspiration. The inter-year variation in CET was larger during the summer season. In the summer, CET was lower in the years in which less precipitation occurred, and a midday depression in the rates of transpiration and evapotranspiration was observed when pF increased. Therefore, water stress was suggested as the cause of inter-year variation in CET. When the effect of water stress was excluded, CET was lower in April, May, and October, and its seasonal variation was around 20%. Since the ratio of transpiration to evapotranspiration was lower in April and May, seasonal depression in CET was suggested to occur due to depressed transpiration that occurred as a result of a small leaf area index (LAI). Thus, finally, we proposed the estimation of daily evapotranspiration more precisely by using the formula 0.97 × f (pF) × g (LAI) × PET, in which the decreasing function f (pF) and the increasing function g (LAI) were introduced.

A sensitivity and error analysis for the penman evapotranspiration model

Meteorological data used in this study were collected over 21 locations in South Korea by the Korea Meteorological Adminstration (KMA) for last 10 years. Using mean values of four meteorological factors (air temperature, wind speed, relative humidity and solar radiation) for the period from January 1, 1991 to December 31, 2000 values of the relative sensitivity were calculated, and error analysis was performed. Relative sensitivity was examined to compare the importance of four meteorological and flux variables on Penman potential evapotranspiration (PET) estimation. The study results show that on the whole, solar radiation is the most sensitive meteorological factor and wind speed is the least sensitive meteorological factor for most of measurement locations. The sensitivity of evapotranspiration rates to changes in humidity is less in the inland areas than in the coastal areas. The evapotranspiration error values indicate that the meteorological factor measurement error is very small compared with the mean daily evapotranspiration.

The effect of rigid scheduling on productivity and water-use of potatoes (Solanum tuberosum L.) grown under small-scale irrigation

This paper evaluates the results of a study highlighting the effect of rigid scheduling on crop water use and soil water extraction patterns, crop coefficients, crop response functions and water-use efficiency (WUE). The cumulative Penman potential evapotranspiration (Etp) at the end of the season reached around 400 mm and the total depth of water applied between planting and harvest through irrigation (I) and rainfall (P) varied from 68 mm in rain-fed treatments to 670 mm in the wettest treatment. Crop water-use (Etc) varied from about 120 mm to around 280 mm corresponding to a daily mean value of Etc at the maximum stage of growth (55 days after planting) from about 1.3 mm to around 5.6 mm for fully irrigated crops. Maximum crop coefficients (Kc) ranged from 0.46 for rain-fed crops to 1.2. Crop yield reached ca. 9.1 t ha-1; for rain-fed treatments and ca. 31.1 t ha-1; for irrigated crops. The relationship between the total tuber fresh weight (t ha-1;) and total water applied (P+I) yielded a polynomial quadratic function (r?=75%). By contrast, the relationship between the total tuber fresh weight (t ha-1;) and total water-use displayed a linear function (r?=66%). The WUE for total water applied ranged from ca. 0.14 t ha-1; mm-1; in rain-fed crops to ca. 0.04 t ha-1; mm-1; for well irrigated crops following a linear function (r?=84%). These results indicate that rigid scheduling under small scale irrigation affects productivity, water-use and water-use efficiency of potatoes with a considerable waste of resources.

The effect of rigid scheduling on productivity and water-use of potatoes (Solanum tuberosum L.) grown under small-scale irrigation

This paper evaluates the results of a study highlighting the effect of rigid scheduling on crop water use and soil water extraction patterns, crop coefficients, crop response functions and water-use efficiency (WUE). The cumulative Penman potential evapotranspiration (Etp) at the end of the season reached around 400 mm and the total depth of water applied between planting and harvest through irrigation (I) and rainfall (P) varied from 68 mm in rain-fed treatments to 670 mm in the wettest treatment. Crop water-use (Etc) varied from about 120 mm to around 280 mm corresponding to a daily mean value of Etc at the maximum stage of growth (55 days after planting) from about 1.3 mm to around 5.6 mm for fully irrigated crops. Maximum crop coefficients (Kc) ranged from 0.46 for rain-fed crops to 1.2. Crop yield reached ca. 9.1 t ha-1; for rain-fed treatments and ca. 31.1 t ha-1; for irrigated crops. The relationship between the total tuber fresh weight (t ha-1;) and total water applied (P+I) yielded a polynomial quadratic function (r?=75%). By contrast, the relationship between the total tuber fresh weight (t ha-1;) and total water-use displayed a linear function (r?=66%). The WUE for total water applied ranged from ca. 0.14 t ha-1; mm-1; in rain-fed crops to ca. 0.04 t ha-1; mm-1; for well irrigated crops following a linear function (r?=84%). These results indicate that rigid scheduling under small scale irrigation affects productivity, water-use and water-use efficiency of potatoes with a considerable waste of resources.

The impact of site factors and climate variability on the calculation of potential evaporation at Moel Cynnedd, Plynlimon

Abstract. The meteorological record from the manual Moel Cynnedd climate station at Plynlimon in the Welsh Uplands has been supplemented with solar radiation data, initially from the Institute of Hydrology's Dolydd Office, and later from an adjacent automatic weather station, in order to calculate Penman potential evapotranspiration for the entire 27 year data set, 1969-1995. The methods of data capture are consistent with Meteorological Office criteria throughout the entire record, establishing an unbiased and probably unique indicator of climatic variability and change for this type of environment. Values of Penman Et calculated from these data provides an independent index of atmospheric demand for moisture as an adjunct to the hydrological studies being carried out in the Plynlimon and neighbouring catchments. Analysis of the long term data indicates considerable year-to year variability in the component variables, including some cyclical changes and possible long term trends in measured temperature. Annual variability in Et is less than in the component variables, and there is an indication of a possible long term cycle, but no evidence of an overall trend in Et during this particular study period. The results indicate that some of the observed variability can be explained by inevitable changes in exposure within this forest clearing site rather than changes in regional or global climatic patterns. A single meteorological station sited in a forest clearing at a relatively low altitude may underestimate potential evaporation across the catchment, as this will also include areas of exposed hillside and forest canopy.

AN EVALUATION OF ACTUAL EVAPOTRANSPIRATION IN TROPICAL EAST AFRICA

Monthly actual evapotranspiration (AET) for four humid catchments in Kenya, East Africa was evaluated using the Morton and Grindley models. The area of each catchment was less then 100 km 2 and all catchments lie around the equator. Three of the catchments are chiefly vegetated with pasture, annual and perennial crops, whereas one is largely under forest. The AET estimates from the aforementioned models were compared with those based on a water balance analysis. A total of 34 data years for daily rainfall and run off for all the catchments were used for analysis. The results indicated that both models tended to overestimate AET in relation to the water balance-based values. The Grindley model (AETG) overestimated such that the estimates were either equal or close to the Penman potential evapotranspiration (PET) values in all the catchments. The Morton model (AET M ) performed better, and AET estimates by this method, although marginally higher, were closer to the water balance-based estimates. The overall overprediction by the Grindley model on a monthly basis was of the order of 32% whereas by the Morton model it was only 8%. Although the mean values from the Morton model are only 8% higher than the water balance values on a monthly basis, values of the RMSE (root mean square error) range between 25 and 47 mm. The additional merit of the Morton model lies in its ability to provide estimates of AET based solely on meteorological data, which are readily available in Kenya, East Africa.

Comparison of Penman‐Monteith, Shuttleworth‐Wallace, and Modified Priestley‐Taylor Evapotranspiration Models for wildland vegetation in semiarid rangeland

Eddy correlation measurements of sensible and latent heat flux are used with measurements of net radiation, soil heat flux, and other micrometeorological variables to develop the Penman‐Monteith, Shuttleworth‐Wallace, and modified Priestley‐Taylor evapotranspiration models for use in a sparsely vegetated, semiarid rangeland. The Penman‐Monteith model, a one‐component model designed for use with dense crops, is not sufficiently accurate (r2 = 0.56 for hourly data and r2 = 0.60 for daily data). The Shuttleworth‐Wallace model, a two‐component logical extension of the Penman‐Monteith model for use with sparse crops, performs significantly better (r2 = 0.78 for hourly data and r2 = 0.85 for daily data). The modified Priestley‐Taylor model, a one‐component simplified form of the Penman potential evapotranspiration model, surprisingly performs as well as the Shuttle worth‐Wallace model. The rigorous Shuttleworth‐Wallace model predicts that about one quarter of the vapor flux to the atmosphere is from bare‐soil evaporation. Further, during daylight hours, the small leaves are sinks for sensible heat produced at the hot soil surface.

How Tree Cover Influences the Water Balance of Mediterranean Rangelands

Dehesa ecosystems of the southwestern Iberian Peninsula are characterized as a savanna—like rangeland dominated by scattered mediterranean evergreen oak trees. We investigated whether isolated trees modify the water balance of this ecosystem and if so, what implications this finding might have on models that assume homogeneity of soil water resources. The water balance of the two ecological components of the dehesas–(1) the tree—grass component, and (2) the open areas between the tree canopies with unshaded grass vegetation–was studied for three consecutive years in three locations in the Sierra Norte de Sevilla region of Andalusia in southern Spain. In this region, annual rainfall was generally between 600 and 800 mm, and the summer drought lasted °130 d. Soil water storage was measured with a neutron moisture gauge outside and under the tree canopy. Deep drainage between two consecutive census dates was calculated using field—measured drainage characteristics. Evapotranspiration (Ea) and surface runoff were computed from the water balance equation assuming that Ea is limited by Penman potential evapotranspiration. Monthly Ea by annual species in open areas was poorly correlated with rainfall levels in the autumn and was limited during the spring by availability of water in the top 40 cm of soil. During summer, monthly Ea by trees ranged from 30 to 50 mm. Mean annual Ea was 400 mm outside and 590 mm under the tree cover. In open areas, water yield (WY), defined as the sum of deep drainage and surface runoff, ranged from 65 to 100% of total Ea, whereas under the tree canopy WY was only 20 to 40% of the Ea. Under the tree canopy, when annual precipitation was &lt;570 mm, WY was negligible and all precipitation was lost by evapotranspiration. Outside the tree canopy, WY occurred as soon as annual precipitation exceed 250 mm. Models of competition between trees and grass generally assume a spatial homogeneity of soil hydrodynamic properties. Our results, however, show that both soil water storage and evapotranspiration are greater for the tree—grass component. Consequently, these models must account for this spatial variability in water resources according to species.

Methodology for Estimating Single Tree Transpiration Based on Heat Pulse Velocity and Micrometeorological Data

A methodology for estimating a single tree transpiration based on a mathematical model which describes the transpiration process with heat pulse velocity and micrometeorological data is proposed. The proposed model consists of a transpiration sub-model and a soil water uptake sub-model, which take account of both the plant physiological activities such as the opening of stomata and the reduction of soil water uptake by roots due to the decline of soil moisture so as to describe the hydrological interaction between soil, vegetation and atmosphere. Model parameters are identified with heat pulse velocity, Penman's potential evapotranspiration and soil moisture potential. The product of Penman's potential evapotranspiration and a transpiration factor defined as the function of the plant water storage is substituted for the actual transpiration rate in the procedure identifying model parameters in order to take the plant physiological properties into consideration. The proposed model was applied to the observed data about a 31 yearold sugi (Cryptomeria japonica) in the Shirakawadani experimental basin in Tokushima prefecture, and estimated the single tree transpiration in the basin.

Computing the water balance of a small agricultural catchment in southern England by consideration of different land-use types. II. Evaporative losses from different vegetation types

Actual evaporative losses have been calculated and used in annual water balances for a small agricultural catchment in southern England. This was done on an areal basis, by calculating the evaporative losses from different land cover types — arable, grassland and forested areas — and applying those losses to the percentage areas of the land cover types, the latter having been obtained by satellite imagery. It was found that this technique improved the annual water balance of the catchment, compared with using Penman potential evapotranspiration for the dominant land use type, particularly during dry years when a substantial soil moisture deficit would limit transpiration losses.

Hydrometeorological differences between opposite valley slopes and vegetation asymmetry in Hawaii

Abstract. Hydrometeorological observations over a 15‐month period and the evaluation of calculated water balance components across a selected valley in the northeast trade‐wind environment in Hawaii showed the following: A large reduction in wind speed and evaporation occurred on the upper protected west‐facing slope as compared with an exposed ridge immediately in front of it and within a short horizontal distance. Can evaporation values measured across the valley closely reflect the wind exposure and the availability of energy at each station. In terms of Penman potential evapotranspiration, the ridge station was again found to be the most exposed and the upper leeward slope the most protected. The upper protected west‐facing slope was comparatively more favourable for vegetation development in terms of the annual soil moisture conditions (both measured and calculated) and protection against mechanical damage by wind and air‐borne salt; all the other parts of the slopes were exposed and occasionally received severe salt spray damage in winter. Most probably, these factors have contributed to the difference in speed of vegetation recovery from the disturbance, thus producing a, temporary, vegetation asymmetry.

An Australian comparison of Penman's potential evapotranspiration estimates and class A evaporation pan data

Estimates of potential evapotranspiration are required to serve as an aid for estimating actual evapotranspiration. Penman's combination equation is generally accepted as an appropriate method for estimating potential evapotranspiration. However, as all the climatic data required to calculate Penman's potential evapotranspiration are seldom available, potential evapotranspiration is more commonly approximated as a factor times standard evaporation pan reading. In this paper, linear regression equations relating Penman's potential evapotranspiration for land surfaces to Class A evaporation pan data over several time periods are developed for various climatic regions throughout Australia. The analyses indicate that the correlations between daily estimates of Penman's potential evapotranspiration and pan data are poor, and therefore, pan data should be treated with caution if used to approximate daily potential evapotranspiration. The correlations improve over longer time periods, and the equations developed for three-day and weekly totals may be used as a last resort to approximate potential evapotranspiration in areas where climatic data required to calculate potential evapotranspiration are not available.

THE APPLICABILITY OF MORTON'S AND PENMAN'S EVAPOTRANSPIRATION ESTIMATES IN RAINFALL‐RUNOFF MODELING1

ABSTRACT: Estimates of the upper constraint on actual evapotranspiration are required as input data in the majority of rainfall‐runoff models. This paper compares and discusses the applicability of Penman's potential evapotranspiration estimates and Morton's wet environment evapotranspiration estimates in rainfall‐runoff modeling applications. Morton's wet environment evapotranspiration depends only on the atmospheric variables and is the estimate of evapotranspiration that would occur when water supply is not limiting. It is a conceptually more correct representation of the upper constraint on actual evapotranspiration compared to Penman's potential evapotranspiration which is dependent on the water supply to the soil‐plant surfaces. Although Penman's potential evapotranspiration and Morton's wet environment evapotranspiration are two different quantities, comparison of the two estimates using data from different climatic regions throughout Australia indicate that they provide similar magnitudes of the upper limit of actual evapotranspiration at moderate climatic conditions when reliable estimates are required in rainfall‐runoff models. The two estimates can therefore be used interchangeably in rainfall‐runoff modeling applications.

Measurement of water use and pasture growth on Templeton silt loam

Abstract Soil water contents to a depth of 1.05 m and pasture growth were measured over 3 years on irrigated and dryland perennial ryegrass-white clover pasture on a Templeton silt loam soil near Christchurch, New Zealand. Cumulative dry matter (DM) yield was linearly related to cumulative Penman potential evapotranspiration when soil moisture deficits in the top 1.05 m of soil were less than 90 mm, and averaged 11.3 kg DM/ha per mm potential evapotranspiration. Growth was slower for soil moisture deficits between 90 and 160 mm, and stopped at greater deficits. The maximum soil moisture deficit obtained in a very dry year was 190 mm. Soil moisture extraction occurred mainly from the top 450–600 mm of soil at low water deficits. As the soil dried, progressively more moisture was taken up from lower down the profile. Yields were reduced at smaller soil water deficits than transpiration.

Seasonal variation in the vertical distribution of white clover stolons in grazed swards

Abstract Swards under mob stocking or rotational grazing by sheep at Kaikohe, Taupo, Ballantrae, Wairarapa, Palmerston North, Lincoln, and Gore were sampled every three months for three years. The white clover stolon material was dissected into three vertical classes: aerial, surface, and buried. Length and dry weight of stolon in each class were measured. The mean total weight of stolon and the distribution by weight of stolon in the three vertical classes varied among sites (P<0.001). In addition, each year there was a highly significant (P<0.001) seasonal change in the vertical distribution of stolon at each site, with mean levels of approximately 80070 of stolon buried in winter and 40% in summer and autumn. At two sites, Wairarapa and Lincoln, exceptions to the seasonal. pattern of distribution of stolon were observed when low rainfall periods occurred. A significant negative correlation between threemonthly effective rainfall (rainfall minus Penman potential evapotranspiration) and weight per unit l...

The wetting and drying of a grazed and ungrazed clay soil

SUMMARYA 4 year field experiment clearly demonstrates the role played by desiccation cracks in the redistribution of rainfall during the autumn re‐wetting of a Windsor Series clay soil. Where topsoil structure is damaged by the trampling of grazing animals, cracks are well developed and a portion of rain water is diverted down them, by‐passing the topsoil. Redistribution takes place from two centres, the bottom of the cracks and the surface. But where structure is unaffected by trampling, the soil accommodates seasonal shrinkage without cracking and redistribution is by diffusion from the surface only. Summer evaporation is shown to be linearly related to Penman's potential evapotranspiration over a wide range of soil moisture deficits, and simple constants are provided to correct ET to obtain EA. The M.A.F.F. method of estimating evapotranspiration is shown to be too conservative and fails to identify the October maximum soil moisture deficit common in this soil.