Storm Runoff Flashcards
What is a hydrograph
A hydrograph represents the evolution in time of the river discharge at a given location along a river. A hydrograph is composed of a baseflow and the storm runoff
What is baseflow
The base flow, is the flow that would have been observed at the given location along a river if the storm had not occurred
What is storm runoff
The storm runoff is flow that is a direct and immediate consequence of the storm
How is baseflow and storm runoff separated
The separation of baseflow and storm runoff first involve finding the beginning and the end of the storm runoff period. The start of the storm runoff coincides with the moment at which the river flow starts to increase after the start of the precipitation. The end of the storm runoff is when a change of regime of the river flow is seen. The determination of the storm runoff period is mostly based on empirical evidence, when flow goes from storm runoff + baseflow to only baseflow there is a change in recession constant. The log of the river flow can be graphed as a function of time to graphically represent this change. The storm runoff is then obtained by subtracting the baseflow from the total flow
What is Hortons Concept
The Horton concept states that storm runoff is caused by surface runoff only, groundwater runoff is considered to be too slow to generate storm runoff. Surface runoff only occurs when the rainfall intensity exceeds the infiltration capacity of the soil so water that doesn’t infiltrate reaches the the river. This definition only applies in specific conditions so a differentiated approach is now used that take into account topography, pedology, land use, vegetation and the climate
What are the processes that generate storm runoff
Saturation overland flow, return flow, lateral flow processes such as subsurface stormflow and unsaturated through flow and groundwater flow
What is saturation overland flow
This occurs in parts of the river basin where soil is saturated throughout its depth and the groundwater table thus reaches the soil surface
What is return flow
Return flow is subsurface flow that returns to the surface which can happen when subsurface flow is forced to the surface by a downslope thinning of the permeable soil layer or where the groundwater table reaches the surface at the border of the unsaturated zone
What are the lateral flow processes and when do they occur
Lateral flow processes occur in the soil above the groundwater table when the surface slope is high and if the soil has a high permeability. Subsurface stormflow may occur during intense rainstorms where the soil becomes saturated at some depth and water migrates laterally through the fissures, rootholes and pores. The unsaturated throughflow may even occur at low rainfall intestines for very permeable soil (soil not saturated).
Groundwater flow
Precipitation infiltrates into the unsaturated upper soil layer and percolates vertically to the saturated zone and subsequently continues its way as a quasi horizontal saturated groundwater flow.
Partial area - variable source concept
Partial-variable source states that only a limited and variable part of the river basin is the source of the storm runoff. The source area is normally situated near the rivers and is determined by geologic, topographic and pedologic factors. The antecedent precipitation (or lack of it) is very important for the extent of the source area, and the concept is believed to be most applicable in humid climates. In the source area, the groundwater table is at a high level, consequently, precipitation will cause rapid saturation of the soil and a consequent surface runoff to the river. Also, precipitation that falls directly on the river and lake surface, return flow and subsurface storm flow can be important. During the storm, the source area further expands due to the rise of the groundwater table. When the storm ends, the surface of the source area will shrink, as a consequence of the decrease of the level of the groundwater table and due to evapotranspiration.
