Lecture 21 Runoff (1) Sources and Components Definition Quick flows and slow flows Sources and components Terms to remember

General Comments Rivers carry excess water from land to oceans. They are important part of global hydrological cycle. The total discharge of river water to oceans approximately equal the total evaporation from oceans River water was the main source of water for humans in early civilization, but other sources of water (ground water, lakes) have been increasingly utilized

Rainfall and Runoff

Runoff Runoff is a process of water movement under the influence of gravity in channels of variable sizes from smallest trickles to the largest rivers. It includes stream flow, overland flow, subsurface flow, and ground water flow. Units: volume per unit time. “Cumec” is one cubic meter per second (m3/s). Cumec per square kilometer ((m3/s)/km2) is often used. To compare with precipitation, runoff is also expressed as depth equivalent over a catchment, i.e., mm/d, mm/m, mm/y, etc.

Terms to Remember Catchment: an area that is responsible for a river or a branch of a river Catchement yield: total runoff from a catchment per unit time. It is the same as other synonyms: stream discharge, or river discharge Hydrograph: the variation of stream discharge with time

Quick and Slow Flows Quickflow: part of the rainfall taking a rapid route to the stream channels, such as overland flow. Delayed flow: part of the discharge which is less directly affected by individual rainfall events but is determined by slow processes such as subsurface flow and ground water flow. It is also called baseflow

Sources and Components of Runoff (1) Channel precipitation Precipitation that directly falls on open water bodies (rivers, lakes, ponds, etc.). An important component of runoff only under these conditions: (1) arid regions where most rainfall over land is adsorbed, and (2) the areal fraction of open water bodies is relatively large.   (2) Overland flow Water that flows over the ground surface to stream channels. It occurs sometimes as quasi-laminar sheet flow, but more often as an anastomosis (network) of trickles and rivulets. Ground Water

Sources and Components of Runoff (3) Throughflow Water that infiltrates the soil surface and then moves laterally towards the stream channels in the upper soil horizons. It moves either in the unsaturated zone (slow), or the perched saturated zone above the main ground water. It is also called subsurface storm flow, storm seepage and secondary baseflow.   Conditions where throughflows occur include: (i) thin permeable soil overlies impermeable bedrock, (ii) the soil layers are distinct, and (iii) there exists an ironpan or ploughpan

Sources and Components of Runoff (4) Ground water flow Water that infiltrates the soil surface and then percolates to the underlying ground water will move laterally to main stream channels through the saturated zone. It contributes mainly to the base flow, but can also be significant in the quick flow through the “piston mechanism”.  

Runoff Sources and Components WR, Figure 7.3