Spatial Distribution of Evapotranspiration in Central Platte River Basin

ABSTRACT

Quantification of spatial distribution of reference evapotranspiration is of importance in

studies involving water and energy balances on earth's surface and is important for numerous

disciplines including water resource planning, management, and distribution, irrigation management,

and ecological, hydrological, and climate modeling. The main objective of this work was to estimate

daily and seasonal spatial distribution of grass reference evapotranspiration (ETo) with FAO-56 PM and

Hargreaves and Samani (H-S) equation using inverse distance weighted (IDW) and spline interpolation

methods with climate data for 2007 in Central Platte River Basin (CPRB) in Nebraska (NE). Overall FAO-

56 PM predicted higher ETo than H-S. Both ETo methods were in good agreement on annual basis (r2 =

0.85), but significant discrepancies were observed between the two methods on growing season basis

(r2 = 0.64). Higher discrepancy was mainly due to high or low relative humidity and wind speed. In

general, ETo increased from eastern part of the basin to the western part with slightly higher ETo in

central part of the basin for both FAO-56 PM and H-S methods, indicating a strong spatial variability.

We also investigated the effect of sequence of steps followed in computing ETo namely interpolate first

and calculate later (IC) and calculate first interpolate later (CI) approach. With the H-S method, there

were no significant differences between CI and IC predicted ETo values when both spline and IDW were

used. However, with the FAO-56 PM method, significant difference was observed between ETo values

with CI and IC approaches when spline interpolation was used. The surrounding stations affected the

ETo prediction directly in CI approach or indirectly in IC approach. The difference between CI and IC

approaches specifically depends on data distribution, and the relationships between climate variables

and underlying processes that control the ETo. Spatial distribution of climate variables, especially wind

velocity and relative humidity, were difficult to model from point measurements. More accurate

spatial modeling of these two parameters can be helpful to recognize the accurate differences in CI

and IC approaches for spatial modeling of ETo. Further research could focus on using more

sophisticated interpolation methods such as kriging interpolation and testing other ETo equations. An

extensive field campaign and spatial data analyses are necessary to determine accuracy of

interpolation techniques and sequence of steps followed in ETo estimation.

STUDY AREA

Figure. The location of automated weather stations (AWSs) in and around the study locations in the Central

Platte River Basin (CPRB) in Nebraska.

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QUALITY CONTROL OF WEATHER DATA

Figure. Examples of quality assessment for AWS data a) Comparison of measured solar radiation with clear sky solar radiation envelope at

Dickens AWS location; b) comparison of solar radiation measured at Clay center location by HPRCC and by BRES with clear sky solar

radiation envelope; c) comparison of daily wind speed Kearney and Smithfield AWS location; d) comparison of daily wind speed Arthur and

Arapahoeprairie AWS location ; e)Comparison of daily Tmin and Tdew at Central City AWS location; f) Comparison of daily Tmin and Tdew at

Lincoln IANR AWS location ; g) Comparison of daily RHmin and RHmax at Cedar Point AWS location; h) Comparison of daily RHmin and RHmax at

Ord AWS location; i) Comparison of measured solar radiation with clear sky solar radiation envelope at Central City AWS location for

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10 Feb - Cloud Cover 92%

15 Apr - Cloud Cover 0% 17 May - Cloud Cover 0%

2 Oct - Cloud Cover - 0%

6 Sept - Cloud Cover – 85%

Cloud cover data at around 10 am

for Landsat path 29 and row 32

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8 Jan - Cloud Cover 0%

24 Jan - Cloud Cover 0%

2 Feb - Cloud Cover 80% 3 March - Cloud Cover 90%

10 May - Cloud Cover - 0%

19 July - Cloud Cover – 50%

5 Sept - Cloud Cover – 0%

Cloud cover data at around 10 am

for Landsat path 30 and row 32

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selected clear sky and cloudy days during 2007 j) Comparison of measured solar radiation with clear sky solar radiation envelope at

Lexington AWS location for selected clear sky and cloudy days during 2007.

SPATIAL DISTRIBUTION OF DAILY REFERENC EVAPOTRANSPIRATION

(a) (b)

Figure. Spatial distribution of growing season ETo from May 1 through September 30, 2007 with IC-IDW

method: (a) H-S and (b) FAO-56 PM.

Figure. Difference between CI and IC approach (CI-IC, mm) on September 30, 2007 (day 273) with FAO-

56 PM: (a) spline and (b) IDW methods.

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ETo (mm)

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ETo (mm)

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ETo (mm)