How To Estimate Evapotranspiration
There are many methods of estimating evapotranspiration.
Some of these are briefly described in the following:
1. Soil Moisture Sampling:
A large number of samples are taken from various depths in the roots zone and the soil moisture depletion studies are made.
2. Tanks and Lysimeter Experiments:
Keeping an account of water added and soil moisture changes by weighing the tanks.
3. Inflow-Outflow Method :
For large areas where yearly inflow into the area, annual precipitation, yearly outflow from the area and the change in ground water level are evaluated.
Small amounts of water are applied at each irrigation to avoid deep percolation.
5. Installation of Colorado Tanks:
The tanks are 90 × 90 × 90 cm made of 1.6 mm mild steel sheets with 2.5 × 2.5 cm angle stiffeners welded to the edges and rims. The tanks, with bottoms for studies of evapotranspiration and without bottoms to include percolation, with crop and without crop inside, are placed in position in a level field with the rim 15 cm above ground level and 7.5 cm standing depth of water inside as shown in Fig. 2.7 . After lowering the tanks inside the ground to the desired level the repacking of the soil excavated has to be done layer by layer in the same order and well compacted.
Lowry-Johnson Equation Used by USBR:
*Ept = cta
P/E Index Method:
u = kf and U = KF = kf f = tp/100
U = ktp/100
k = kt kc
Et = kEp
Evapotranspiration Soil Moisture Snow Fire Weather And Sea Level Rise
Increased potential evapotranspiration throughout Australia is simulated to occur in the future with increased temperature, including the regions in the MDB. The magnitude of change by 2030 is around a 2%6% increase . This increase in potential evapotranspiration will result in a tendency for reductions in soil moisture on average in the MDB that are stronger than one may expect from rainfall changes alone . The gap between water demand and water supply will therefore also increase under climate change.
Also, fire weather conditions would become harsher, and the window for back burning to control fuel load will be reduced, primarily due to increased temperature. These changes are also influenced by rainfall changes and would be most evident in areas with reduced rainfall .
Higher temperatures will lead to a reduced likelihood of precipitation falling as snow and faster melt of any snow on the ground. The CCIA concluded with high confidence that snowfall and maximum snow depth would decline. This would have the effect of shifting the period of peak runoff to earlier in the season in snow-affected catchments .
Sea level rise at the Murray mouth, estimated from the relevant regional sea level rise projections in CCIA, shows projected increase by 2030 at Victor Harbour of 0.12 m under RCP4.5 and 0.13 m under RCP8.5 . The CCIA does not provide regional projections for sea level rise in 2050, but by 2090 the increases will be 0.45 m under RCP4.5 and 0.60 m under RCP8.5 .
Et Estimated From Pan Evaporation
Evaporation from an open water surface provides an index of the integrated effect of radiation, air temperature, air humidity and wind on evapotranspiration. However, differences in the water and cropped surface produce significant differences in the water loss from an open water surface and the crop. The pan has proved its practical value and has been used successfully to estimate reference evapotranspiration by observing the evaporation loss from a water surface and applying empirical coefficients to relate pan evaporation to ETo. The procedure is outlined in Chapter 3.
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Characterization Of Drought Properties
In this case study, we have used the SPEI for drought analysis that was also embraced recently for detecting drought onsets and terminations, drought ranking, and recurrence evaluation . Using total rainfall and the reference evapotranspiration data, the SPEI on a 3-month timescale was calculated for the present case study location. Because the SPEI was a standardized index, the value SPEI = 0 corresponded to the mean with respect to the base period 19712000 and the SPEI = ±1 corresponded to the standard deviation where the negative SPEI indicated dry condition.
In accordance with the SPEI time series representing the deficits and surpluses of water resources relative to a well-defined base period, the drought onsets and terminations were then identified in periods when the SPEI declined to a value below zero . The drought duration D, S and I properties were thus estimated from the SPEI time series via the widely adopted run-sum approach described in the study by Yevjevich :
where j = 1 is the start of a drought event when the SPEI drops below zero and its continuation as a negative value for at least 3 months to the length n, D = total duration from the onset to the termination period, while the intensity, I = the drought peak for a given drought event.
Pet For Different Land Cover Types Across A Latitudinal Transect
Do different PET models produce equivalent PET estimates? The answer will vary depending on the spatial and temporal scales, as well as the land cover. For example, the three PET models that we compared at the 11 sites performed similarly at some sites, but diverged greatly at the majority of the sites . Generally, the models differed from each other in average annual PET by more than 25%. For instance, the Thornthwaite and PenmanâMonteith models differed by 1% at the permanent wetland site and 6% at the closed shrubland site. But the Thornthwaite model almost always estimated smaller values of PET than did the PriestleyâTaylor and PenmanâMonteith models. This difference suggests that either the PriestleyâTaylor and PenmanâMonteith models are over-estimating, or that there may be more PET occurring than estimated by the Thornthwaite model, which is dependent on temperature and day length alone, due to other factors such as atmospheric control. The PenmanâMonteith model often gave the highest estimates .
Differences in mean annual potential evapotranspiration estimates between the
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Runoff: Meaning Types And Factors
In this article we will discuss about:- 1. Meaning of Runoff 2. Types of Runoff 3. Factors Affecting Runoff 4. Runoff Cycle 5. Computation.
Meaning of Runoff:
Rainfall is the primary source of water for runoff generation over the land surface. In common course of rainfall occurrence over the land surface, a part it is intercepted by the vegetations, buildings and other objects lying over the land surface and prevent to reach them on ground surface, called interception. Some part of rainfall is also stored in the surface depressions, referred as depression storage, which in due course of time gets infiltrated or evaporated.
When all these losses are satisfied, then excess rainfall moves over the land surface and reaches to the smaller rills, known as overland flow. The overland flow again builds a greater storage over the land surface and draining the same into channels/streams is termed as runoff.
Thus, runoff may be defined as that portion of rainfall as well as any other flow, which makes its way towards the river, stream or oceans etc. Since, runoff is through the channel, stream/or rivers etc., therefore, sometimes it is also called as channel flow.
Based on the time delay between the instance of rainfall and generation of runoff, the runoff may be classified into following three types:
1. Surface Runoff:
2. Sub-Surface Runoff:
3. Base Flow:
1. Type of Precipitation:
2. Rainfall Intensity:
3. Duration of Rainfall:
4. Rainfall Distribution:
Examples Of Evapotranspiration In A Sentence
evapotranspiration The Arizona RepublicevapotranspirationForbesevapotranspirationNational GeographicevapotranspirationAnchorage Daily Newsevapotranspiration Scientific AmericanevapotranspirationScientific AmericanevapotranspirationCBS NewsevapotranspirationThe Economist
These example sentences are selected automatically from various online news sources to reflect current usage of the word ‘evapotranspiration.’ Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.
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How Do You Measure Throughflow
Measuring throughflow Dig a pit in the soil, to whatever depth you would like to investigate, and place the plastic guttering at the base of the pit. It is even possible to dig a series of steps at different depths, and use a length of guttering at each depth to measure throughflow at different depths within the soil.
What Is Infiltration And Aquifer
Infiltration seepage of atmospheric precipitation through a porous aeration zone is studied for assessing the values of groundwater recharge or natural resources. Penetrating of atmospheric precipitation into an aquifer by mountain rock fracturing, through karst sinkholes and pores is called influation.
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What Are The Factors Influencing Evapotranspiration
In addition to solar radiation, air temperature, humidity, soil water content, etc., are some other factors that influence transpiration. When soil water availability decreases, so does transpiration. During drought, when the topsoil is dry, the plants can begin to wilt. Transpiration also depends on the type of the plant. Plants in dry areas have adaptations that reduce transpiration, unlike trees and plants in wetter areas. The geography of a place, namely its latitude and climate, can also influence evapotranspiration. Evapotranspiration is higher at locations close to the Equator. If the area receives little rainfall, then evapotranspiration can reach 100% of the total precipitation as in the southwestern United States.
Change In Soil Moisture Storage
Water that is added or removed from the water stored in the soil results in a change in soil moisture storage. This change in soil moisture storage can lie in between the lower limit of 0, and the upper limit of the field capacity.
Soil moisture deficit is when potential evapotranspiration is greater than actual evapotranspiration. In other words, it is when the demand for water goes beyond what is actually available. Deficit only occurs when the soil contains no moisture at all. This is when soil moisture storage is at 0. Determining the deficit allows one to discover how much water is needed to to balance out the system.
Surplus occurs when precipitation exceeds the amount necessitated by potential evapotranspiration . It is when the amount of water present in an environment is beyond what it actually needs. In a surplus, the soil is at its field limit and can no longer accommodate the water in excess. The surplus water would instead run off the soils surface and be deposited in nearby water sources such as streams. This increases the volume of water in a stream, which in turn would also lead to an increase in discharge. Determining the surplus can aid in predicting the flooding of streams in the areas vicinity.
An area of land that drains surface run-off into a common point is called a watershed.
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Guide To Choosing An Et Model
From examination of the literature on species diversity gradients cited herein, it is rarely the case that authors provide a clear argument for their choice of ET data. To some extent we surmise that research teams frequently choose data on the basis of the ease of availability. With so many ET models to choose from, how should one select the model and data that will be the most appropriate to the task in hand?
There are two simultaneous and potentially contradicting goals when choosing a model : accurate prediction and simplicity . The most accurate model may be the most complex model, but this assumes that the input data are completely available and accurate also, the complexities of such models may not be very important â in other words, ET at a specific study site may be sensitive to only a few of the many potential controlling factors. Above all, the model must be appropriate for the study system. Therefore, two critical factors must be assessed in choosing a model: the quality and availability of the input data, and the sensitivity or degree of response of ET to the potential controls for the geographical area of the study system.
- ET, evapotranspiration AET, actual evapotranspiration PET, potential evapotranspiration NDVI, normalized difference vegetation index EVI, enhanced vegetation index NDWI, normalized difference water index.
Relative Importance Of Evapotranspiration
- evapotranspiration represents nearly all the soil and surface water loss from dry environments, where runoff is minimal
- on a global scale, evapotranspiration consists mostly of evporation from soil, because a large proportion of the continents is occupied by desert grassland and tundra
- with a continuous vegetation cover, evaporation from soil is least important and transpiration and evaporation of intercepted water account for most of the ET
- there vegetation has a profound impact on the hydrological cycle, such that streamflow can be augmented by managing vegetation
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Management And Environmental Conditions
Factors such as soil salinity, poor land fertility, limited application of fertilizers, the presence of hard or impenetrable soil horizons, the absence of control of diseases and pests and poor soil management may limit the crop development and reduce the evapotranspiration. Other factors to be considered when assessing ET are ground cover, plant density and the soil water content. The effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. On the other hand, too much water will result in waterlogging which might damage the root and limit root water uptake by inhibiting respiration.
FIGURE 4. Reference , crop evapotranspiration under standard and non-standard conditions
Where field conditions differ from the standard conditions, correction factors are required to adjust ETc. The adjustment reflects the effect on crop evapotranspiration of the environmental and management conditions in the field.
How Do Plants And Animals Cope With Evapotranspiration
Plants possess the ability to monitor the amount of water lost by transpiration. The short-term mechanism is to close the stomata. Some plants even keep their stomata closed during the day when the rate of transpiration is higher. In addition, they can have many physical features that help them cope with heat or dryness in the environment. In hot and dry deserts, the cacti leaves are small and reduced to hard spines with fewer stomata. The stem is thick and flattened with a hard and green wall which produces food by photosynthesis. The hard waxy wall prevents any evaporation. Among animals, camels in the Sahara can survive for six to seven months without drinking water, as they make do with water in their food. Animals also tend to rest in the shade in the afternoon to avoid the sun and are active in mornings or evenings. In temperate Mediterranean regions, the summers are dry. Here, annuals grow and complete their cycle to avoid the dry summer. Some trees are deciduous, losing their leaves in summer. Evergreen trees have tough leaves with a special coating to prevent evaporation.
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Expanding The Geography Of Evapotranspiration: An Improved Method To Quantify Land
Affiliation Wyler Department of Dryland Agriculture, French Associates Institute for Agriculture and Biotechnology of Drylands, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer, Israel
Roles Conceptualization, Data curation, Formal analysis, Methodology, Supervision
Affiliation Soil and Environment Studies Program, Federal University of Paraná, Curitiba, Paraná, Brazil
Definition Of Potential Evapotranspiration
Rosenberg says that potential evapotranspiration is the evaporation from an extended surface of short green crop which fully shades the ground, exerts little or negligible resistance to the flow of water, and is always well supplied with water. Potential evapotranspiration cannot exceed free water evaporation under the same weather conditions.
In fact, we know that real ET differs from the potential under most circumstances. The reasons for these differences are best explained by reference to the conditions imposed by the definition of PET and by an analysis of the reality of these conditions. We follow the discussion of , even though we could use other references .
C.D. Woodroffe, A.C. Falkland, in, 2004
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Controls On Evaporation Rate
Annual Sums Of Actual Evapotranspiration
The shortfall in moisture utilization when actual evapotranspiration is less than potential evapotranspiration provides a practical index to the moisture health of an ecosystem over a season or year. This index is sometimes applied in the form of a difference, or sometimes a ratio. The difference is that between actual and potential evapotranspiration, i.e., the kilograms per square meter of water that, if it had been available at the right time, would have been evaporated by the ecosystem this number serves as the basis for estimating irrigation requirements. The ratio is that of actual to potential evapotranspiration, i.e., 0.60 in the median year at Blue Canyon, in the California Sierra .
Both the difference and the ratio forms are useful in analyzing crop yields . A small difference or shortfall, or a high ratio, indicate that moisture stress was generally small or absent, although of course such other environmental factors as nutrient availability also affect crop yields. One general depiction of the relation between the actual-potential evapotranspiration ratio and the actualpotential yield ratio is shown in Fig. XII-8 . This graph further illustrates the idea that moisture stress late in the growing season reduces yield less than stress coming early, during the phase of active vegetative growth when high transpiration is a concommitant of rapid photosynthesis.
Dennis L. Hartmann, in, 2016
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