Crop yield response to water - Cra

densities. This range is referred to in this writing as full canopy cover and the time when thisis reached is referred to as canopy closure. However, these terms are used rather loosely in theliterature, and can be referred to CC substantially less than 0.95, as low as 0.9 or even lower insome writings. As a general guide, CC is 0.95 or higher when LAI exceeds about 4.5 or 5.0 (Figure1), with some exceptions. One exception is species exhibiting strong sun-tracking behaviour,such as sunflower, which requires an LAI of 3.5 to achieve full canopy cover. Another exceptionis if the crop is planted in clumps, or very close to each other in rows that are spaced widelyapart. In this case LAI significantly higher than 5.0 is necessary to achieve full canopy cover.As plant density is reduced below a particular level, the density is insufficient for the canopyto close and CC falls substantially below 0.95. This point depends on the kind of crop, eachwith its particular limit of potential leaf area per plant. Ideally, for each kind of crop a curve ofCC x vs. plant density, based on experimental data, should be constructed for use in AquaCropsimulations. Unfortunately, for some species, the required experimental data are lacking. Ifthe user has CC x measurements of his/her crop at the plant density in question under optimalgrowth conditions, these CC x values are obviously the best to use in the simulation. OtherwiseCC x needs to be estimated. One way is simply visual, judging the extent of CC by eye aroundthe time when CC is maximum. A word of caution here, viewing the canopy from the side oreven at a downward angle (or photograph taken from similar positions) tends to overestimatethe CC because this view may include too many plant layers. Viewing the canopy from directlyabove, or viewing the proportion of the soil shaded by the canopy when full sun is directlyoverhead, is the better way to make the estimate. Estimates can also be made based on generalknowledge of the crop or similar crops.If the CC x of a particular crop is known for a particular plant density (reference planting), toestimate CC x of a planting of the same kind of crop but planted at a different density (d p ), onecan start by estimating the maximum LAI of the planting of known CC x (LAI ref ) from Figure1 (or a similar relationship if more accurate), and calculating first the LAI of the plantingassuming leaf area per plant is independent of plant density, then make a rough adjustmentfor the impact of change in plant density. This is summarized as an equation:⎛dp⎞ ⎛(1) LAI = LAI ref ⎜ ⎟ ⎜⎝ d ref ⎠ ⎝1F adj⎞⎟⎠where d ref is the plant density of the reference, and F adj is the adjustment factor.F adj is limited between the range of d/d ref and 1.0, for cases where d p > d ref , as well as whered < d ref . To illustrate, first take the case of d p > d ref . If the d p /d ref = 1.3, F adj would be limitedto the range of 1 to 1.3. At one extreme where F adj = 1, the leaf area per plant would beindependent of plant density. At the other extreme where F adj = 1.3, the leaf area per plant isreduced by the increase in plant density so much that the LAI of the planting remains the sameas LAI ref . In the case of d p < d ref , if d p /d ref = 0.7, F adj would be limited to the range of 1 to 0.7.Obviously, in most cases, the limit values of F adj should not be used to estimate LAI. The extentthe plant adjusts its leaf area in response to crowding is related to how determinate the cropis in growth habit. So the more indeterminate the crop is, the more F adj should deviate from1.0, either smaller or larger.80crop yield response to water