| dc.description.abstract | Reliable estimates of the percentage of the soil surface covered by crop residues and
shaded by crop canopies are required for the management of soils to reduce erosion.
The importance of ground cover is reflected in its role in erosion prediction models
as the cover-management factor. Research has shown that detachment of soil
particles by raindrops is the prerequisite to erosion, and that ground cover is the
greatest deterrent to this process through raindrop interception and attenuation. There
also occurs a critical cover value below which erosion is significant and above which
it is insignificant.
Most techniques of estimating ground cover are not very precise. Consequently,
conservation options derived from predictive models are likely to be erroneous due
to inaccurate cover management factors. This research was therefore meant to
evaluate and calibrate four different cover measurement techniques (i.e. the
photographic method, the sighting-frame. the ceptometer, and the metre-stick
method). These techniques were evaluated on maize, bean and maize-bean intercrop
cover in agro-c1imatic zone III (semi-humid).
All pairs of technique comparisons gave very high correlation coefficients (r) in the
range of 0.95 - 0.99. This suggested that any of the four methods could be used for
routine ground cover estimation. Although all the techniques gave higher estimates
of cover than the actual cover, calibration of the crop cover values against the
physical plant models gave r-values of 0.921, 0.909, 0.689, and 0.467 for the
camera, sighting frame, metre stick, and ceptometer respectively. The cover
XlI
correction factors obtained from the regression equations were 0.88, 0.875, 0.799,
and 0.752 respectively. Consequently, the accuracy of cover estimation was highest
for the camera and lowest for the ceptometer. The sighting-frame gave better cover
estimates than the metre- stick. Precision was highest for the camera and ceptometer.
This was attributed to their high objectivity contrary to the sighting frame and metrestick
which are more exposed to subjective bias. Analysis of variance showed that all
the techniques were independent and there was no interaction between any technique
and the crop. The sighting frame was however recommended for routine field
application for row crops due to its cost effectiveness, local availability, ease of use
and reI iable accuracy.
Two approaches in the use of the Sighting frame were also compared; the
conventional 0-0.5-1 and the new 0-0.25-0.5-0.75-1 systems. Both approaches had
r-values in the range of 0.95 - 0.99. Consequently any of the two systems could be
adopted for routine cover evaluation. A preliminary rainfall energy interception model
for the short rains 1992/93 at the research site was also suggested. Of importance
were the effects of extreme rainfall events realised towards late 1992 and early 1993.
It was recommended that for all cover measurement techniques, evaluation procedures
standardization and design improvements be made to increase the precision and
accuracy of cover estimation and to facilitate ease of application in the field. | en |
