GIS based calculations for Lake/Water surface evaporation in the state of Texas.

By:Paramjit Chibber

Texas A & M University.

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Boundary_tx.shp# Ev_98.shp

Reservoirs

PROBLEM DEFINITIONPROBLEM DEFINITION

Index

1)Importance of evaporation in Reservoirs.

2)Data collection and completion.

3)Methodology involved.

4)Limitations.

Importance of Evaporation.

Def : Process by which water changes from the liquid or the solid state into

gaseous state. (Ref:Merrill Bernard et. Al.).

Storage :-. 1)Wide shallow reservoirs. 2)Deep narrow reservoirs.

Note : Effects more prominent in tropical countries where annual. Evaporation sometimes exceeds Precipitation.

Measurement of water surface evaporation.

Pan Evaporation Method :A circular pan of 4 Ft. diameter and 10 inchesdeep and is exposed on a wooden platform.

Eo = cEp.

Eo =Lake Evaporation.C = Pan evaporation coefficient.Ep = Pan evaporation.

Data Collection and Completion.

Datasets Used:

1)Pan Evaporation at Gage Stations (NWS Hydrologic Research Lab).

2)Pan Evaporation in different Quadrangles (TWDB).

3)Reservoirs and Counties Datasets (TNRIS).

4)Pan Evaporation Coefficients (TWDB)

Raw Data

NAME=DAINGERFIELD 9 S

LAT= 32.92 LON= 94.72 ELEV= 300

<- 1960 -> <- 1970 -> <- 1980 -> <- 1990 ->

01234 56789 01234 56789 01234 56789 01234 56789

778C9 99C8C CC898 C9652 89-74 59878 86-85 75487 JAN

C88C8 89CC8 CCCC9 7C983 87677 47764 978-6 87779 FEB

DATA COMPLETION

After this for all gage stations complete the datasets (Ep) for the year 1968.

Following the same procedure complete the datasets for other years i.e 1958, 1978,1988,and 1998 as well.

Now in excel spread sheet do the evaporation calculations at all gage stations using pan evaporation coefficients.

Up till last step we have calculated the water surface evaporation at all gage stations.

NOW :Add Texas boundary theme.Add gage stations theme.OBSERVATION : One gage station found to be out side theboundary.

SOLUTION : Buffer theboundary of Texas.

Methodology

1) Thiessen polygons: In this method polygons of same evaporation are generated

2) Grid method : In this method grids are created to measure evaporation.

1. Thiessen polygons: Create thiessen polygons with gage stations, considering buffered outer boundary.

Note:Same reservoirs can have different evaporations.

Now Intersect “reservoirs” with the “thiessen polygons”.Update Areas of the intersected reservoirs.Add field Ev(AcFt).Fill up the Ev(AcFt) field.

Ev(AcFt) = Area*Annual Evaporation (in inches) *247.1057/12000000.

Note : After this step Annual evaporation of all

the intersected polygons is known.

Now

Dissolve the intersected polygons (taking Reservoir# as the key attribute).

Look at the statistics for Ev(Acft) field.

Sum Ev(AcFt) gives annual evaporation.

Following the same steps find out evaporation for other years.

Year : 1958 1968 1978 1988 1998

Evaporation (Acre Ft) : 11475752 11462713 14315370 12117721 12029697

0

2000000

4000000

6000000

8000000

10000000

12000000

14000000

16000000

1958 1968 1978 1988 1998

Evaporation (AcreFt)

2) Grid Method :This method converts point data into a grid or

surface data.

Now “Summarize zones” to get the mean evaporation (in inches)

Add Field Ev_gr(AcFt).

Fill the Ev_gr(AcFt) with

Ev_gr(AcFt) = Mean*Area*247.1057/12000000.

“Statistics “of Ev_gr(AcFt) inform of the Annual evaporation.

Following the same steps find out evaporation for other years.

Year 1958 1968 1978 1988 1998

Evaporation (AcFt) : 12008612 10766627 12574776 12174579 12070145

9500000

10000000

10500000

11000000

11500000

12000000

12500000

13000000

1958 1968 1978 1988 1998

Evaporation (AcreFt)

Limitations. With Grid method it is observed that we can sometimes get wrong data.The possible reasons might be :

Data not available at all gage stations.Original dataset incomplete.