Contents preface (VII) introduction 1—37

Maximum depths of rainfall for accumulated areas in mm (Example 2.3)

2	13	12.22	10.64
4	25	21.92	19.76
6	34	29.69	26.27

It should be noted that the DAD curves need not be straight line as seen in Fig. 2.11 and the area axis may be logarithmic.

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rainfall(mm) Maximumaveragedepthof

IRRIGATION AND WATER RESOURCES ENGINEERING

40

30
						6 hrs
20						4 hrs
10						2 hrs
0
0	1000	2000	3000	4000	5000	6000

Area (km²)

Fig. 2.11 DAD curves for Example 2.3
2.3.8. Mean Annual Rainfall
Mean annual rainfall for a given basin/catchment/area is computed as the arithmetic average of total yearly rainfall for several consecutive years. Mean annual rainfall obtained from rainfall records of about 30–40 years is expected to be true long-term mean annual rainfall with an error of about 2% and is acceptable for all types of engineering problems.
2.3.9. Probable Maximum Precipitation (PMP)
Probable maximum precipitation (PMP) is that magnitude of precipitation which is not likely to be exceeded for a particular basin at any given time of a year in a given duration. Thus, PMP would yield a flood which would have virtually no risk of being exceeded in that duration. Obviously, such a precipitation would occur under the most adverse combination of hydrological and meteorological conditions in the basin/area. Estimation of PMP is useful for obtaining the design flood for the purpose of designing hydraulic structures such as spillways failure of which would result in catastrophic damage to life and property in the surrounding region.
PMP can be estimated (6) using either meteorological methods (7) or statistical studies of rainfall data. One can derive a model (for predicting PMP) based on parameters (such as wind velocity and humidity etc.) of the observed severe storms over the basin and then obtain the PMP for maximum values of those parameters. Alternatively, one can also estimate the PMP by adopting a severe storm over a neighbouring catchment basin and transposing it to the catchment/basin under consideration. PMP estimates for North-Indian plains vary from about 37 to 100 cm for one-day rainfall.

2.4. ABSTRACTIONS FROM PRECIPITATION
Prior to rain water reaching the watershed outlet as surface runoff or stream flow, it has to satisfy certain demands of the watershed such as interception, depression storage, evaporation and evapotranspiration, and infiltration.
A part of precipitation may be caught by vegetation on the ground and subsequently get evaporated. This part of precipitation is termed intercepted precipitation or interception loss (which, incidentally, is the gain for the atmospheric water) which does not include through-fall (the intercepted water that drips off the plant leaves to join the surface runoff) and stemflow (the intercepted water that runs along the leaves, branches and stem of the plants to reach the

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57

ground surface). Interception loss primarily depends on storm characteristics, and type and density of vegetation.

Part of precipitation which fill up all the depressions on the ground before joining the surface runoff is called depression storage. It depends primarily on (i) soil characteristics, (ii) magnitude of depressions on the ground and (iii) antecedent precipitation which would decide soil moisture level.
Evaporation is the physical phenomenon by which a liquid is tranformed to a gas. The rate of evaporation of precipitation depends on (i) the vapour pressure of water, (ii) prevailing temperature, (iii) wind speed, and (iv) atmospheric pressure. Transpiration is a phenomenon due to which water received by the plant through its root system leaves the plant and reaches the atmosphere in the form of water vapour. Evaporation and transpiration are usually considered together as evapotranspiration (or consumptive use), Art. 3.7.
Infiltration is the passage of water from the soil surface into the soil and is different than percolation which is the gravity flow of water within the soil. Infiltration cannot continue unless percolation removes infiltered water from the surface soil. The maximum rate at which a soil can absorb water at a given time is known as infiltration capacity, f_c, expressed in cm/ hour. The actual (prevailing) rate of infiltration f at any time is expressed as

	f = fc	if i ≥ fc
and	f = i,	if i < fc

where, i is the intensity of rainfall. Infiltration capacity depends on several factors such as soil characteristics including its moisture content, and vegetation or organic matter. Porosity is the most important characteristics of soil that affects infiltration. Forest soil, rich in organic matter, will have relatively higher infiltration capacity, largely because of the corresponding increase in porosity. Also, infiltration capacity for a given soil decreases with time from the beginning of rainfall primarily because of increasing degree of saturation of soil. Therefore, it is obvious that the infiltration capacity of a soil would vary over a wide range of values depending upon several factors. Typical values of f_c for sand and clay would be about 12 mm/h and 1.5 mm/h, respectively. A good grass cover may increase these values by as much as 10 times.

Difficulties in theoretical estimation of infiltration capacity due to its complexity have led to the use of infiltration indices. The simplest of these indices is the φ -index defined as the rate of rainfall above which the rainfall volume equals the runoff volume. This means that other initial losses (such as due to interception, evaporation and depression storage) are also considered as infiltration. The φ-index can be obtained from the rainfall hyetograph, Fig. 2.12. On this hyetograph is drawn a horizontal line such that the shaded area above this line equals the measured runoff.

Rainfallintensity(cm/h)

2.5
2.0
1.5												Runoff
1.0
0.5					Losses							f-index
0
	1	2	3	4	5	6	7
0

Time (h)
Fig. 2.12 φ-index

2.5. RUNOFF

Precipitation (or rainfall), after satisfying the requirements of evapotranspiration, interception, infiltration into the ground, and detention storage, drains off or flows off from a catchment

58 IRRIGATION AND WATER RESOURCES ENGINEERING
basin as an overland flow (or surface runoff which includes precipitation falling on the stream system too) into a stream channel. Some part of the infiltrating water moves laterally through the upper layers of the soil and returns to the ground surface as interflow or subsurface runoff at some place away from the point of infiltration into the soil. Part of the infiltrating water percolates deep into the ground and joins the ground water storage. When water table intersects the stream channels of the catchment basin, some ground water may reach the surface or join the stream as ground water runoff, also called base flow or dry-weather flow. Thus, the runoff from a catchment includes surface runoff, subsurface runoff and base flow. The surface runoff starts soon after the precipitation and is the first to join the stream flow. Subsurface runoff is slower and joins the stream later. Depending upon the time taken by the subsurface runoff between the infiltration and joining the stream channel, it may be termed as prompt subsurface runoff or delayed subsurface runoff. The groundwater runoff is the slowest in joining the stream channel but, is responsible in maintaining low flows in the stream during dry season. Based on the time interval between the precipitation and runoff, the runoff is categorized as direct runoff (that enters the stream immediately after precipitation i.e., surface runoff and subsurface runoff) and base flow (i.e., ground water runoff). Runoff, thus is the response of a catchment to the precipitation reflecting the combined effects of the nature of precipitation, other climatic characteristics of the region, and the physiographic characteristics of the catchment basin.

Type, intensity, duration and areal distribution of precipitation over the catchment are the chief characeteristics of the precipitation that affect the stream flow. Precipitation in the form of rainfall is quicker to appear as stream flow than when it is in the form of snow. For the surface runoff to start, the intensity of rainfall (or precipitation) must exceed the infiltration capacity of the soil which decreases with the increase in the duration of rainfall. It is, therefore, obvious that a longer duration rainfall may produce higher runoff even if the intensity of rainfall is less but, of course, exceeding the infiltration capacity of the soil. Heavy rainfalls in the downstream region of the catchment will cause rapid rise in the stream levels and early peaking of the discharge. A rare occurrence of uniformly distributed rainfall may result in increased infiltration and, therefore, increased subsurface runoff and base flow resulting in slow rise in levels and delayed peaking of the discharge. Likewise, antecedent higher soil moisture conditions at the time of precipitation would hasten the rise in the stream levels.

Other climatic characteristics influencing the runoff are temperature, wind velocity, and relative humidity. These characteristics affect the evapotranspiration and thus influence the availability of the precipitation for runoff. Physiographic characteristics affecting the runoff have been discussed in Art. 2.7.