Contents preface (VII) introduction 1—37

Table 2.1 Barlow’s runoff coefficient K_b in per cent (Developed for use in UP)

Class		Description of catchment		Values of Kb (per cent)
	Season 1		Season 2	Season 3
A		Flat, cultivated and absorbent soils	7			10	15
B		Flat, partly cultivated, stiff soils	12		15	18
C		Average catchment	16		20	32
D		Hills and plains with little cultivation	28		35	60
E		Very hilly, steep and hardly any cultivation	36		45	81

Season 1 : Light rain, no heavy downpour

Season 2 : Average or varying rainfall, no continuous downpour
Season 3 : Continuous downpour
Strange analysed the rainfall and runoff data for the border region of Maharashtra and Karnataka and expressed runoff in terms of rainfall as (8)

R = KsP

(2.14)

in which the runoff coefficient K_s depends on catchment characteristics only as given in Table 2.2.
Table 2.2 Strange’s runoff coefficient K_s in per cent
(For use in border areas of Maharashtra and Karnataka)

Total monsoon		Runoff coefficient Ks (per cent)
rainfall (cm)	Good Catchment		Average Catchment	Bad Catchment
25	4.3		3.2	2.1
50	15.0		11.3	7.5
75	26.3		19.7	13.1
100	37.5		28.0	18.7
125	47.6		35.7	23.8
150	58.9		44.1	29.4

For Western Ghats, Inglis and De Souza gave (8) R = 0.85 P – 30.5

Similarly, for Deccan plateau, they gave
1
R = ₂₅₄ P (P – 17.8) In both these relations, R and P are in cm.

Khosla analysed monthly data of rainfall P_m (cm), runoff (°C) for various catchments of India and USA to obtain (9)

R_m = P_m – L_m

with R_m ≥ 0 and L_m = 0.48 T_m for T_m > 4.5°C. Here, L_m represents monthly losses in cm.

(2.15)

(2.16)
R_m (cm) and temperature T_m

(2.17)



60 IRRIGATION AND WATER RESOURCES ENGINEERING
For T_m ≤ 4.5°C, the monthly loss may be assumed as follows:

	Tm°C	4.5	– 1	– 6.5
	Lm (cm)	2.17	1.78	1.52
12
Annual runoff = ∑ Rmi
	i = 1
This formula is found to give fairly good estimates of runoff.
Using hydrologic water-budget equation, runoff R can be expressed as
	R = P – Eet – ∆S		(2.18)

where, E_et is actual evapotranspiration and ∆S is the change in soil moisture storage. Both these parameters depend upon the catchment characteristics and the regional climatic conditions. With the help of available data and computers (to handle large mass of data), one can develop a mathematical relationship (i.e., model) incorporating interdependence of parameters involved. This watershed model is, then, calibrated i.e., the numerical values of various coefficients of the model are determined using part of the data available. The remaining available data are used for validation of the model. Once the model is validated satisfactorily, it becomes a handy tool to predict the runoff for a given rainfall.
Another technique that can be used for computer simulation of watershed is artificial neural network (ANN) which is being increasingly employed for predicting such quantities which cannot be expressed in the form of mathematical expressions due to inadequate understanding of the influence of all the factors that affect the quantities.
2.6. STREAM FLOW
2.6.1. Flow Characteristics of a Stream
The flow characteristics of a stream depend upon (i) the intensity and duration of rainfall besides spatial and temporal distribution of the rainfall, (ii) shape, soil, vegetation, slope, and drainage network of the catchment basin, and (iii) climatic factors influencing evapotranspiration. Based on the characteristics of yearly hydrograph (graphical plot of dis-charge versus time in chronological order), one can classify streams into the following three types:
(i) Perennial streams which have some flow, Fig. 2.13, at all times of a year due to considerable amount of base flow into the stream during dry periods of the year. The stream bed is, obviously, lower than the ground water table in the adjoining aquifer (i.e., water bearing strata which is capable of storing and yielding large quantity of water).

Discharge
2	4	6	8	10	12
			Time (Months)			Dec

Fig. 2.13 Temporal variation of discharge in perennial streams

HYDROLOGY

61

(ii) Intermittent streams have limited contribution from the ground water and that too during the wet season only when the ground water table is above the stream bed and, therefore, there is base flow contributing to the stream flow, Fig. 2.14. Except-ing for some occasional storm that can produce short duration flow, such streams remain dry for most of the dry season periods of a year.

Discharge

2 4 6 8 10 12

Dec

Time (Months)
Fig. 2.14 Temporal variation of discharge in intermittent streams
(iii) Ephemeral streams do not have any contribution from the base flow. The annual hydrograph, Fig. 2.15, of such a stream shows series of short duration hydrographs indicating flash flows in response to the storm and the stream turning dry soon after the end of the storm. Such streams, generally found in arid zones, do not have well-defined channels.

Discharge

2 4 6 8 10 12

Dec

Time (Months)

Fig. 2.15 Temporal variation of discharge in ephemeral streams
Streams are also classified as effluent (streams receiving water from ground water storage) and influent (streams contributing water to the ground water storage) streams. Effluent streams are usually perennial while the influent streams generally remain dry during long periods of dry spell.