You know that the forest vegetation that covers 120 km^2 wildland watershed uses

Question

You know that the forest vegetation that covers 120 km^2 wildland watershed uses an average of 4mm of water per day druing the summer. You also know that the soil averages 150cm in depth and at field capacity can store 0.2mm of water per mm of soil depth. However, at the beginnning of the season the soil was dry, holding an average of only 0.1mm/mm. As the watershed manager, you are asked to predict how much water will be carried by the streams draining the watershed during the 90-day summer period when 450mm of precipitation falls on the area. Use the balance water equation to make a rough prediction of the stream discharge as a percentage of the precipitation and in cubix meters of water.

Explanation / Answer

Stream discharge is normally expressed in units of volume per unit time (e.g., cubic metres per second), although this is sometimes converted to an equivalent depth over the upstream catchment area. There are a number of techniques for measuring stream discharge. Measurements of velocities using current meters or ultrasonic sounding can be multiplied by the cross-sectional area of flow. Dilution of a tracer can also be used to estimate the total discharge. Weirs of different types are frequently employed at discharge measurement sites. These are constructed so as to give a unique relationship between upstream water level and stream discharge. Water levels can then be measured continuously, usually with a float recorder, to construct a record of discharge over time—namely, a stream hydrograph. Analysis of the hydrographic response to catchment inputs can reveal much about the nature of the catchment and the hydrologic processes within it.

Stream discharge data are presented in terms of daily, monthly, and annual flow volumes, though for some purposes (e.g., flood routing) shorter time periods may be appropriate. The frequency characteristics of peak discharges and low flows are also of importance to water resource planning. These are analyzed using some assumed probability distribution in a way similar to rainfalls. A time recording of annual maximum flood is usually used in flood-frequency analysis. For design purposes the hydrologist may be asked to estimate the flood with a recurrence interval of 50 or 100 years or longer. There are few discharge records that are longer than 50 years, so such estimates are almost always based on inadequate data.

Knowledge of the discharge characteristics of catchments is essential to water supply planning and management, flood forecasting and routing, and floodplain regulation. Discharges vary over short lengths of time during storm periods, seasonally with the seasonal changes in evapotranspiration losses, and over longer periods of time as the rainfall regime changes from year to year. Discharge characteristics also vary with climate. In some places discharge represents only a minor component of the catchment water balance, the losses being dominated by evapotranspiration.

The discharge hydrograph that results from a rainstorm represents the integrated effects of the surface and subsurface flow processes in the catchment. Traditionally, hydrologists have considered the bulk of a storm hydrograph to consist of storm rainfall that has reached the stream primarily by surface routes. Recent work using naturally occurring isotope tracers such as deuterium has shown, however, that in many humid temperate areas the bulk of the storm hydrograph consists of pre-event water. This water has been stored within the catchment between storms and displaced by the rainfall during the storm. This suggests that subsurface flow processes may play a more important role in the storm response of catchments than has previously been thought possible.

Underlying all the hydrologic sciences is the concept of water balance, an expression of the water cycle for an area of the land surface in terms of conservation of mass. In a simple form the water balance may be expressed as

S = P Q E G,

where S is the change of water storage in the area over a given time period, P is the precipitation input during that time period, Q is the stream discharge from the area, E is the total of evaporation and transpiration to the atmosphere from the area, and G is the subsurface outflow. Most hydrologic studies are concerned with evaluating one or more terms of the water balance equation. Because of the difficulties in quantifying the movement of water across the boundaries of an area under study, the water balance equation is most easily applied to an area draining to a particular measurement point on a stream channel. This area is called a catchment (or sometimes a watershed in the United States). The line separating adjacent catchments is known as a topographical divide, or simply a divide. The following sections describe the study of the different elements of the catchment water balance and the way in which they affect the response of catchments over time under different climatic regimes.

water balance may be expressed as S = P Q E G,

Here we want discharge Q = P S E G,

from the given data P=450/90=50 mm

                                     E = 0.1mm

                                     G=4 mm

                                     Depth = 150 cm = 1500 mm now L = 750 area S= 562 mm

substiyute all values in above eq we get discharge Q= 516 m3 / sec

Related Questions

Navigate

• Browse (All)
• Browse Y
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.