Lecture 22 Runoff (2)

Event-based Hydrographs• Hortonian hypothesis

• Hewlett hypothesis

Event-based Hydrographs

Separating the sources of water flow affecting hydrographs in individual events is the basis for understanding the runoff processes

Horton HypothsisRainfall on the soil surface is partitioned into two components: infiltration and overland flow. The overland flow results in the quick portion of the streamflow, and water after the infiltration contributes to the delayed flow or is evaporated. The overland flow is called infiltration-excess overland flow.

Horton HypothesisThe infiltration capacity reduces during a rainfall event Horton hypothesis is most suitable for• Areas devoid of vegetation• Arid and semi-arid areas• Cultivated fields In most vegetated humid regions, the Horton hypothesis may not apply. In these regions, infiltration capacity is high, thus overland flow is small. The quickflow in these cases may result from subsurface flows and ground flows.

Hewlett Hypothesis

All rainfall at a location infiltrates to the soil until it is saturated and the water table rises to the surface level. Overland flow occurs only after the complete soil saturation. So it is called saturation overland flow

Hewlett Hypothesis

Flow in disjunct source area

These are areas where subsurface flow convergence occurs. After the convergence, the overland flow appears, or the flow is transmitted through hydrological linkages. They are caused by

• Slope concavities in plan• Slope concavities in cross-section• Areas where the soil becomes thinner than the upslope• Areas of reduced hydraulic conductivity

Hydrological linkages

WR, Figure 7.3

Hydrological linkages allow the rapid transmission of water from the disjunct areas to the stream channels.  Forms of the linkages include• Macropores• Soil pipes

Role of Throughflow

The maximum rate of through flow is about 5-6 m/d, so the simple calculation for rainfall to reach the stream channels from upslope areas would suggest that the throughflow can not contribute significantly to the quick flow. However, this may not be true.

Role of Throughflow

Thatched Roof Analogy

Role of Throughflow

“Piston displacement”: New water added at one location forces old water at lower heights to emerge. This is a mechanism that can happen instantaneous through changes in the water potential distribution in the system. Throughflow can therefore contribute to the quickflow in a very short time.This piston mechanism can only work when the soil is saturated or

near saturated

Role of Ground WaterGroundwater generally moves slowly and contribute insignificantly to storm flows, but there are many situations where groundwater flows can play an important part in storm hydrographs. These situations may include:

• Shallow water table, where ground water and surface water is interlinked, e.g., The Netherlands

• Groundwater ridge adjacent to the stream channel

• Conditions for the piston displacement