Pond Drainage Area and Evaporation

Rainfall supply projections as of 12/13/2019

Water Surface is 0.5 acres or approximately 21,800 SF

• 21,800 SF at an average 3.5 foot depth = 648,500 gallons.
• 21,800 x 0.08 = 1,744 CF or 14,800 gallons per inch of depth

Rough rainfall runoff area = 4.7 ac

A 1″ rain over 4.7 ac would be 139k gallons total. If only half runs to the pond then 69,500 gallons would add to the pond 69500/14800 or 4.7 inches.

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Evaporation Rates

From NOAA Technical Report NWS 34 Mean Monthly, Seasonal, and Annual Pan Evaporation for the United States

All of the evaporation values in these tables represent estimates of expected evaporation occurring from a Class A pan …. The evaporation from shallow lakes and moist soils is generally classified by one of the following equivalent names: free water surface evaporation (FWS), lake evaporation (E1 ), or potential evapotranspiration (PE).

An estimate of FWS which is more accurate than that given by multiplying the pan value by 0.70, is obtained by multiplying the pan amount by the appropriate coefficient from map 4 of the evaporation atlas described earlier. [See analysis below.]

Analysis using data from NOAA Technical Report NWS 33 Evaporation Atlas for the Contiguous 48 United States Washington, D.C. June 1982

Our Pond Volume at 160,000 gallons/ft = 13,300 gallons/inch
From NWS 33 – Annual Class A Pan Avg. of 15 years Evaporation in Fairhope AL From Table 2, inches	51
NWS 33 Map 4 coefficient for May-Oct	0.76

	Monthly Percent of Annual Evaporation from NWS 33 Table 2 (1)	Projected Evaporation Inches/Month	76% adj for shallow ponds	Gallons Evaporated from Our Pond	Pump hours per month at 20 GPM to Offset Evaporation w/o Rain
Jan	3.7	1.9	1.4	19,074	16
Feb	4.8	2.4	1.9	24,744	21
Mar	7.8	4.0	3.0	40,210	34
Apr	9.8	5.0	3.8	50,520	42
May	12.5	6.4	4.8	64,439	54
Jun	12.5	6.4	4.8	64,439	54
Jul	12.3	6.3	4.8	63,407	53
Aug	11.1	5.7	4.3	57,221	48
Sep	9.3	4.7	3.6	47,942	40
Oct	7.6	3.9	2.9	39,179	33
Nov	4.8	2.4	1.9	24,744	21
Dec	3.8	1.9	1.5	19,589	16
		TOTAL	38.8

(1) Class A Pan, Station in Fairhope 2NE, AL.