WATER BALANCE

3.13 WATER BALANCE

The input items into a basin are essentially precipitation (P) and subsurface inflow (G i ) while the water losses are evaporation (E), evapotranspiration (E t ) and subsurface outflow (G o ). The

balance goes to recharge ground water (G r ), increase the soil moisture (SMA) and as surface runoff (streamflow, R).

The water balance equation can be written as

P +G i =E+E t +G 0 + SMA + G r +R

A case history of the water balance studies of the Krishna River basin, south India, is as follows:

Water Balance Study of Krishna River Basin

The Krishna river Basin lies between the latitudes 13° 0′ N and 19° 30′ N and longitudes 73°23′ E and 80°30′ E, with a drainage area of 258948 km 2 and total length 1400 km in south India. The hydrometeorological studies and water balance of the basin was made by Subramanyam et al. (1980) employing the book keeping technique of Thornthwaite. 60 Sta- tions were used for studying the rainfall patterns of the basin both on an annual basis and in

different seasons. Estimation of the water losses from the basin by evapotranspiration was made by using Thornthwaite’s formula* for 21 stations in the basin for which the temperature data were available. The monthly normal water balance for the whole year for the Krishna River basin is given in Table 3.2, prepared by using the water balances of individual stations representative of the different sections of the basin (Fig. 3.16).

The water balance study shows that the water need (1375 mm) is higher than the water supply by precipitation (1134 mm), though an amount of 182 mm of water is recorded as rain- fall excess (stream flow plus ground water storage (in underground formations)), on account of concentration of rainfall from June to October compared to the lower values of water need during these months.

*Thornthwaite formula: E pt = ct a , mm/month, where t = mean monthly temperature, °C; c, a are constants depending upon the climatic conditions of the area, latitude and the month, for specific crops.

HYDROLOGY

mm/month E pt P E t SMU

SMA SMA 191 mm 191 mm (G+R) (G+R) 182 182

Fig. 3.16 Water balance of Krishna river basin (after Subramanyam et al. 1980)

Water deficiency (SMD) obtained from the water balance studies indicates the amount of water needed for supplemental irrigations in agricultural operations, adjustment of crop calendar (so that harvest will precede drought) and crop rotation to improve the soil structure and increase the soil moisture storage capacity. Crop yields can increase if the moisture defi- ciency could be avoided.

Table 3.2 Water balance for Krishna river basin (Fig. 3.16)

SMA Month

=P–E pt –(R + G r ) R + G r

(mm) (mm) Jan

(mm)

(mm)

86 6 45 39 41 — — Feb

88 4 31 27 57 — — Mar

118 9 30 21 88 — — Contd.

WATER LOSSES

191 182 Note: E pt = potential evapotranspiration when there is unlimited water in the root zone, i.e., P ≥ E pt .

P = precipitation (mm/month) E t = actual evapotranspiration (mm/month) limited to the availability of water by precipitation

and soil moisture stored; E t ≤E pt SMU = Soil moisture utilisation (mm/month) from storage SMD = Soil moisture deficit (mm/month) = E pt –E t SMA = Soil moisture accretion (mm/month) when P > E pt

R+G r = Rainfall excess (stream flow + Ground water accretion) (mm/month) =P–E pt —SMA; after soil recharge = P—E pt

QUIZ III

I Choose the correct statement/s in the following: 1 The various water losses are (i) the subsurface outflow from the basin (ii) evaporation from ground water (when the GWT is very near the ground surface) (iii) soil evaporation (iv) interception by plant leaves and buildings (v) evaporation from soil and transpiration from plant leaves in an irrigated land (vi) all the above 2 Interception loss is

(i) more towards the end of a storm (ii) more at the beginning of a storm (iii) uniform throughout the storm (iv) high in the beginning of storm and gradually decreases 3 Evaporation from water surface

(i) is proportional to the deficit of vapour pressure (ii) increases with temperature (iii) increases with humidity

HYDROLOGY (iv) increases with the exposed area

(v) increases if there is salinity or pollution (vi) decreases with wind velocity (vii) increases when a film of acetyl alcohol is spread over the surface (viii) is high in arid region 4 Evapotranspiration depends upon (i) hours of bright sunshine

(ii) wind speed

(iii) temperature

(iv) humidity

(v) type of crop

(vi) season of crop

(vii) stage of growth for a given crop (viii) moisture level in the soil (ix) method of irrigation

(x) all the above factors

5 Infiltration occurs at capacity rate (i) if there had been antecedent rainfall (ii) if the intensity of rainfall is lower than the capacity rate (iii) if the intensity of rainfall is higher than the capacity rate (iv) if the rainfall intensity is equal to the capacity rate (v) during a first flash storm following summer (vi) due to watershed leakage (1. except i; 2. ii, iv,

3. i, ii, iv, viii ; 4. x; 5. iii, iv, v)

II Match items in ‘A’ with itemts in ‘B’ (more than one item in B may fit)

(i) Evaporation

(a) Irrigated Land

(ii) Infiltration rate

(b) Plant leaves

(iii) Evaporation opportunity

(c) Humid day

(iv) Low evaporation

(d) φ-index

(v) Evapotranspiration

(e) Soil evaporation

(vi) Transpiration

(f) Rate of entry of rain water into soil (g) Dalton’s law (h) Land pan (i) Float pan (j) Blaney-Criddle formula

III Say ‘true’ or ‘false’; if false, give the correct statement: (i) Land pan can be placed directly over the land, in the vicinity of a lake or reservoir, to measure pan evaporation. (ii) Evaporation is significant in arid regions, while transpiration is significant in humid regions. (iii) Evaporation is less on a humid day. (iv) Evapotranspiration is often used synonymously with the consumptive use. (v) Potential evapotranspiration is the evapotranspiration from a cropped land under limited water supply to the roots. (vi) Measurement of transpiration losses can only be made on small laboratory samples. (vii) The value of φ-index depends only on the soil and is independent of the storm pattern.

WATER LOSSES

(viii) For the same storm of the same duration, total rain and runoff (net rain), there may be different φ-indices for the different storm patterns (i.e., for different time distribution of rain- fall).

(ix) Supra-rain-curve technique yields exact values for hourly and total net rains. (false: i, v, vii, ix)