Cover Image
close this bookHandbook for Agrohydrology (NRI)
View the document(introduction...)
View the documentAcknowledgments
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close this folderChapter 1: Introduction
View the document1.1 The role of hydrology in agriculture
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View the document1.3 Project planning and practical problems
close this folderChapter 2: Measurement of runoff
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View the document2.1 Estimates of runoff
View the document2.2 Collecting runoff data
View the document2.3 Water level recording instruments
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View the documentAppendix A: Measurement of runoff
close this folderChapter 3: Erosion and sedimentation data
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View the document3.1 Soil erosion
View the document3.2 Field measurement of sediments (eroded material)
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View the documentAppendix B: Erosion and sedimentation data
close this folderChapter 4: Rainfall and other meteorological data
View the document4.1 Rainfall
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close this folderChapter 5: Soils and soil moisture data
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View the document5.1. Soil classification and soil textures
View the document5.2. Soil moisture
View the document5.3 Infiltration
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View the documentAppendix C: Soils and soil moisture
close this folderChapter 6: Catchment characteristics
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View the document6.1 Natural vegetation
View the document6.2 Interception
View the document6.3 Catchment size, slope and topography
View the document6.4 Field orientation
View the document6.5 Antecedent soil moisture conditions
View the document6.6 Other catchment influences
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close this folderChapter 7: Water harvesting and field structures
View the document7.1 Water harvesting
View the document7.2 The design of bunds, channels and other field structures
View the document7.3 Surveys, marking out in the field and construction
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View the documentAppendix D1: Bund dimensions for various areas, slopes and soil types
close this folderChapter 8: Data analysis
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View the document8.1 Statistical methods and data analysis
View the document8.2 Non-statistical analysis of agrohydrological data
View the documentAppendix E: Data analysis

6.2 Interception

Interception can only be loosely defined as a catchment characteristic as it is the combined effect of several influential factors such as rainfall, climate and vegetation cover. However, in other respects it falls conveniently into this chapter and so is discussed here.

Losses from interception, the rainfall that collects on vegetation and is re-evaporated, can be highly variable and depends mostly on vegetation type (size, shape and disposition of leaves and branches); rainfall amount, intensity and drop size; wind speed, temperature and eddying. Interception is difficult to measure, especially for crops. It can be attempted by placing rain gauges under vegetation either randomly to sample average interception, or by the selection of specific target areas. In wooded catchments, rain gauges should be attached to tree trunks to assess stem flow, as in Figure 6.6 below, but with multi-stemmed vegetation this is very difficult.


Figure 6.6: Stemflow Measurement on trees

In Figure 6.6, in addition to free-standing gauges under the canopy, a peripheral collector is wrapped around the trunk to direct flow into a single rain gauge that is covered.

Empirical work has led to estimates of losses of 10 - 20% of seasonal rainfall and deduced storage capacities of 0.8 to 1.5 mm of rain per storm. Equation (6.1) describes an empirical interception relation and Table 6.3 gives examples for various crops for a 25 mm rainfall.

I = (Si + Etr) (1 - e-kP) where (6.1)

I = total interception
Si = storage capacity per unit of the area
E = evaporation rate
tr = duration of rainfall
P = amount of rain k = 1/ (Si + Etr)
e = base of natural logs.

In terms of runoff studies, the situation regarding interception is even more complex. It is usually lumped with rainfall storage due to ponding and infiltration for runoff modelling purposes, where it is assigned a purely notional value.

Table 6.3 Interception Losses from a 25 mm Rainfall

Crop

Height (m)

Interception (mm)

Maize

1.8

0.8

Cotton

1.2

8.4

Tobacco

1.2

1.8

Small grains

0.9

4.1

Meadow Grass

0.3

2.0

Alfalfa

0.3

2.8