Contents preface (VII) introduction 1—37

2.2. HYDROLOGIC CYCLE
The total water of earth, excluding deep ground water, is in constant circulation from the earth (including oceans) to atmosphere and back to the earth and oceans. This cycle of water amongst earth, oceans, and atmospheric systems is known as hydrologic cycle.
Figure 2.1 is an enormously simplified sketch of the hydrologic cycle for which sun is the source of energy. The hydrologic cycle (Fig. 2.1) can be visualized to begin with the evaporation (due to solar heat) of water from the oceans, streams and lakes of the earth into the earth’s atmosphere. The water vapour, under suitable conditions, get condensed to form clouds moving with wind all over the earth’s surface and which, in turn, may result in precipitation (in the form of rain water, snow, hail, sleet etc.) over the oceans as well as the land surface of the earth. Part of the precipitation, even while falling, may evaporate back into the atmosphere. Another part of the precipitation may be intercepted by vegetation on the ground or other surfaces. The intercepted precipitation may either evaporate into the atmosphere or fall back on the earth’s surface. The greater part of the precipitation falling on the earth’s surface is retained in the upper soil from where it may return to the atmosphere through evaporation and transpiration by plants and/or find its way, over and through the soil surface as runoff, to stream (or river) channels and the runoff thus becoming stream flow. Yet another part of the precipitation may penetrate into the ground to become part of the ground water. The water of stream channels, under the influence of gravity, moves towards lower levels to ultimately meet the oceans. Water from ocean may also find its way into the adjoining aquifers. Part of the stream water also gets evaporated back into the atmosphere from the surface of the stream. The ground water too moves towards the lower levels to ultimately reach the oceans. The ground water, at times, is a source of stream flow.

38

HYDROLOGY				39
			Precipitation
	Atmospheric system	Evaporation
			Precipitation
			Evaporation	O
			S
Precipitation			Evapotranspiration
				C
			T
Evaporation	Land system	Surface runoff	Stream
				flow
			R	E
			Exfiltration
	Infiltration
			E
				A
Evaporation	Subsurface system	Subsurface runoff	Tidal flow
				A
Precolation				Upward movement of	N
			ground water
			M
			Ground water runoff
	Aquifer	system	Stream water to aquifer
	Sea water intrusion
			Ground water joining ocean

Fig.2.1 Hydrologic cycle
The description of the hydrologic cycle should not lead one to conclude that there is a continuous mechanism through which water moves steadily at a constant rate. The movement of water through the cycle is evidently variable, both in time and space although the total water resources of the earth remains invariant since the formation of the earth system. Further, the hydrologic cycle is a very complex phenomenon that has been taking place since the earth formed. It should also be noted that the hydrologic cycle is a continuous recirculating cycle with neither a beginning nor an end.
Hydrologic system is defined as a structure or volume in space surrounded by a boundary that receives water and other inputs, operates on them internally, and produces them as outputs

40 IRRIGATION AND WATER RESOURCES ENGINEERING
(1). The global hydrologic cycle can be termed a hydrologic system containing three subsystems : the atmospheric water system, the surface water system, and the subsurface water system. Another example of the hydrologic system is storm-rainfall-runoff process on a watershed. Watershed (or drainage basin or catchment ) is a topographic area that drains rain water falling on it into a surface stream and discharges surface stream flow through one particular location identified as watershed outlet or watershed mouth. The term ‘watershed’ used for the catchment area should be distinguished from the watershed used in the context of canal alignment, chapter-5.

2.3. PRECIPITATION
The atmospheric air always contains moisture. Evaporation from the oceans is the major source (about 90%) of the atmospheric moisture for precipitation. Continental evaporation contributes only about 10% of the atmospheric moisture for precipitation. The atmosphere contains the moisture even on days of bright sun-shine. However, for the occurrence of precipitation, some mechanism is required to cool the atmospheric air sufficiently to bring it to (or near) saturation. This mechanism is provided by either convective systems (due to unequal radiative heating or cooling of the earth’s surface and atmosphere) or by orographic barriers (such as mountains due to which air gets lifted up and consequently undergoes cooling, condensation, and precipitation) and results into, respectively, convective and orographic precipitations. Alternatively, the air lifted into the atmosphere may converge into a low-pressure area (or cyclone) causing cyclonic precipitation. Artificially induced precipitation requires delivery of dry ice or silver iodide or some other cloud seeding agent into the clouds by aircrafts or balloons.
The common forms of precipitation are drizzle or mist (water droplets of diameters less than 0.5 mm), rain (water drops of size between 0.5 mm and 6.0 mm), snow (ice crystals combining to form flakes with average specific gravity of about 0.1), sleet (rain water drops, falling through air at or below freezing temperatures, turned to frozen rain drops), and hail (precipitation in the form of ice balls of diameter more than about 8 mm). Most of the precipitation, generally, is in the form of rains. Therefore, the terms precipitation and rainfall are considered synonymous. Rainfall, i.e., liquid precipitation, is considered light when the rate of rainfall is upto 2.5 mm/hr, moderate when the rate of rainfall is between 2.5 mm/hr and about 7.5 mm/hr, and heavy when the rate of rainfall is higher than about 7.5 mm/hr.