School of Environmental Engineering Universiti Malaysia Perlis TAT

School of Environmental Engineering Universiti Malaysia Perlis TAT 901/3 Hydrology

HYDROLOGIC PROCESS Hydrologic cycle Atmospheric Circulation Precipitation Evaporation Evapotranspiration

Hydrologic Cycle Hydrological cycle is the central focus of hydrology. The cycle has no beginning or end, and its many processes occur continuously. PROCESS FLOW: Water evaporates from the oceans and the land surface to become part of the atmosphere. Water vapor is transported and lifted in the atmosphere until it condense and precipitates on the land or the oceans.

Precipitated water may be intercepted by vegetation, become overland flow over the ground surface, infiltrate into the ground, flow through the soil as subsurface flow, and discharge into streams as surface runoff. Much of the intercepted water and surface runoff returns to the atmosphere through evaporation.

The infiltrated water may percolate deeper to recharge groundwater, later emerging in springs or seeping into streams to form surface runoff, and finally flowing out to the sea or evaporating into the atmosphere as the hydrologic cycle continues. (Refer Figure 1 for further details)

Figure 1: Hydrologic cycle with global annual average water balance

Table from World Water Balance and Water Resource of the Earth, Copyright, UNESCO, 1978 Table 1: Estimated world water quantities

Table from World Water Balance and Water Resource of the Earth, Copyright, UNESCO, 1978 Table 2: Global annual water balance

Figure 2: Block-diagram representation of the global hydrologic system

Problems Question : Estimate the residence time of global atmospheric moisture

Hint Use the following formula: where: Tr = Average duration for a water molecule to pass through a subsystem of the hydrological cycle S = Storage Q = Flowrate

Atmospheric Circulation Many meteorological processes occur continuously within the atmosphere. The processes of precipitation and evaporation are the most important for hydrology. Much of the water precipitated on the land surface is derived from moisture evaporated from the ocean and transported long distances by Atmospheric Circulation.

The two basic driving forces of atmospheric circulation result from 1) The rotation of the earth 2) The transfer of heat energy between equator and the poles

The earth constantly receives heat from the sun through solar radiation and emits heat through re -radiation/back radiation into space. These processes are in balance at an average rate of approximately 210 W/m 2.

The heating of the earth is uneven. Near the equator, the incoming radiation is almost perpendicular to the land surface and averages about 270 W/m 2. Near the poles, the incoming radiation strikes the earth at a more oblique angle at a rate of about 90 W/m 2.

The rate of radiation is proportional to the absolute temperature at the earth’s surface, which doesn’t vary greatly between the equator and the pole. The earth emitted radiation is more uniform than the incoming radiation.

If the earth were a nonrotating sphere, atmospheric circulation would appear as in figure. Air would rise near the equator and travel in the upper atmosphere toward the poles, then cool, descend into the lower atmosphere, and return toward the equator. This is called Hadley Circulation.

One cell atmospheric circulation pattern for a nonrotating planet.

The rotation of the earth from west to east changes the circulation pattern. 1) The radius of the ring of air decreases. 2) In order to maintain angular momentum, the air velocity increases with respect to the land surface, thus producing an air flow.

The effect producing these changes in 1) wind direction 2) velocity is known as The Coriolis Force.

The actual pattern of atmospheric circulation has 3 cells in each hemisphere 1) Tropical Cell 2) Polar Cell 3) Middle Cell Latitudinal cross section of the general atmospheric circulation.

Heated air ascends at the equator, proceeds toward the pole at upper level, loses heat and descends toward the ground at latitude 30 o. Near the ground, one branch moving toward the equator and the other toward the pole.

Air rises at 60 o and flow toward the pole at upper levels, then cool and flows back to 60 o near the earth surface. The middle cell is driven by the other two; its surface air flow toward the pole, producing prevailing westerly air flow in the mid-latitudes.

The atmosphere is divided vertically into various zone. 1) Troposphere 2) Stratosphere 3) Ionosphere

The atmospheric circulation occurs in the troposphere. The troposphere ranges in height from about 8 km at the poles to 16 km at the equator. The temperature in the troposphere decreases with altitude at a rate varying with the moisture content of the atmosphere.

The tropopause separates the troposphere from the stratosphere. Near the tropospause, sharp changes in temperature and pressure produce strong narrow air current known as Jet Stream with speeds ranging from 15 -50 m/s. They flow for thousands of kilometer and have an important influence on air mass movement.