|Morrison Jr, John|
Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/1998
Publication Date: N/A
Interpretive Summary: Soil conservation practices for reducing erosion by water include retaining a soil cover of crop residues or vegetative canopy. Measures of residue cover or of green canopy cover are often used to confirm compliance with contract management plans. An accurate (correct) and precise (small range between individual observations) measure is needed for acceptable operation of such a monitoring program. The cover measuring technique most frequently used in fields is an equal spaced point transect (a cord with equally spaced knots or beads) which is set at a 45 degree angle with respect to any rows or stripes caused by tillage. This is quite adequate if residue is uniformly spread over a field. However, when residue cover or a seedling canopy of a row crop is heavily concentrated into a dense stripes with parallel, nearly bare rows between them, there is an interaction of the repeating geometric patterns of the transect points and the rows in the field. This interaction can cause a very large range in estimates of cover in the field. Such imprecise measures make it difficult to say with certainty how much cover exists in a field. At least two solutions are available to obtain precise measures of cover even when the cover is concentrated in stripes. Transects with either random spaced points or sinewave spaced points can provide accurate and precise measures of cover. Unfortunately, random spaced points require extra observations to obtain a desirable level of precision. The sinewave spacing is unique and provides precise estimates from about the same number of points used with equal spaced transects.
Technical Abstract: Linear point transects with equal-spaced points are used in agricultural fields to measure the per cent of the soil surface covered by crop residue or shaded by a crop canopy. When used on uniformly distributed residue, relatively precise measures of per cent cover can be found with as few as five, 100 point counts. Conversely, if an equal-spaced point transect is used to measure cover that lies in regularly spaced parallel rows, the resulting estimates of per cent cover may be highly variable (standard deviation may exceed the mean). The problem is most pronounced when the canopy or residue forms equal-spaced parallel stripes with alternating high (50% or more) and low (20 % or less) percent cover. Precise means (standard deviation less than 5 percentage points) can be obtained with equal-spaced point transects in fields with crop canopy or residue in regularly spaced rows if both point spacing and transect angle relative to tillage or planting rows are custom selected for each row spacing. However, no one equal spacing of points is available that will provide precise cover estimates at all planter and tillage tool row spacings used in agricultural fields. Cover estimates from random-spaced point transects are not dependent upon transect angle or row spacing. If random-spaced points are used along a transect, 100 point counts will have a standard deviation between 5 and 10 percentage points. A sinewave-like point spacing pattern also produces cover estimates that are not dependent upon transect angle or row spacing. The advantages of this pattern are: it is unique in contrast to the large number of possible random spacings, it can be used on any row spacing, and it usually provides a variance less than that obtained from random-spaced points.