FIGURE 1: 30 TAC §210.24(b)

TABLE 1

WATER BALANCE EXAMPLE

(All Units are Inches of Water per Acre of Irrigated Area)

a b Ri c d e f g
Month Average
Precipi-
tation 		Average
Runoff 		Average
Infil-
trated
Rainfall 		Evapo-
transpi-
ration 		Required
Leaching 		Total
Water
Needs
(5)+(6) 		Effluent
Needed
in Root
Zone
(7)-(4) 		Evapo-
ration
from
Reservoir
Surface 		Effluent
to be
Applied
to Land
(8)/K
Consumption
from
Reservoir
(9) + (10)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Jan. 2.11 0.40 1.71 0.80 0.00 0.80 0.00 0.02 0.00 0.02
Feb. 2.43 0.57 1.86 1.20 0.00 1.20 0.00 0.01 0.00 0.01
Mar. 2.02 0.36 1.66 2.80 0.20 3.00 1.34 0.09 1.58 1.67
Apr. 3.19 1.03 2.16 3.40 0.22 3.62 1.46 0.05 1.72 1.77
May 4.19 1.74 2.45 6.10 0.64 6.74 4.29 0.10 5.05 5.15
June 3.30 1.10 2.20 6.50 0.76 7.26 5.06 0.20 5.95 6.15
July 2.20 0.45 1.75 6.70 0.87 7.57 5.82 0.34 6.85 7.19
Aug. 2.12 0.41 1.71 4.60 0.51 5.11 3.40 0.34 4.00 4.34
Sept. 3.58 1.30 2.28 5.10 0.50 5.60 3.32 0.19 3.91 4.10
Oct. 3.09 0.96 2.13 4.10 0.35 4.45 2.32 0.14 2.73 2.87
Nov. 2.23 0.46 1.77 2.10 0.06 2.16 0.39 0.07 0.46 0.53
Dec. 2.34 0.52 1.82 1.00 0.00 1.00 0.00 0.03 0.00 0.03
32.80 9.30 23.50 44.40 4.11 48.51 27.40 1.58 32.25 33.83

Table 1 Footnotes

  1. Up-to date rainfall and evaporation data sets are available from the Texas Natural Resources Information System.

  2. Runoff should be determined by an acceptable method such as the Soil Conservation Service method found in SCS Technical Releases No. 55. For calculation purposes only, a CN value of 74 was assumed for good pasture with Class "C" soils.

  3. Suggested source of values is the "Bulletin 6019, Consumptive Use of Water by Major Crops in Texas", Texas Board of Water Engineers.

  4. In low rainfall areas, this is the required leaching to avoid salinity build-up in the soil where:

    L = Ce__ (E - Ri) Ri = Infiltrated rainfall
    C1-Ce
    Ce = Electrical conductivity C1 = Maximum Allowable Conductivity
    of effluent of Soil Solution (Table 3)
    E = Evapotranspiration

    For calculation purposes only, Ce is measured to be 1.5 millimhos/cm @ 25° and C1 is 10.0 (Bermuda Grass)

  5. Net evaporation from reservoir surface. For the purpose of calculation, an assumption must be made as to the ratio of irrigated land area to reservoir surface area. For this example problem, the necessary reservoir area was assumed to be 17% of the irrigated area. If, after all calculations are made, the reservoir dimensions do not seem reasonable, then a new assumption must be made and the calculations repeated. Values in column (9) are adjusted to be inches per irrigated acre.

  6. K is the irrigation efficiency which for this example is taken to be 0.85.

  7. The total of this column together with the expected annual volume of effluent will determine the acreage of irrigated land required.