Surveys in this category focus on the stages of development of the crop, and the canopy development specifically within the FMU throughout the cropping season. Various parameters can be measured, and we discuss the important ones below. The frequency at which such information should be collected depends, among other things, on the crop type, the parameter being measured, availability of labor vis-à-vis measurement equipment, and the frequency of satellite image acquisition. A default frequency to collect information on crop canopy development is once every 2 weeks. Ideally, field campaign visits coincide with satellite image acquisitions to-the-day at least, but in practice this is hard to achieve as frequent image acquisitions are tasked orders for which no date can be known in advance. (Detrimental cloud cover or other tasks may prevent the task being carried out on a target date.) As a consequence, one should aim at a frequency of field visits that allows the measured parameters to be piecewise linearly interpolatable. Per-visit costs are however often so prohibitive that compromises need to be found.
Parameter measurements can be time-consuming and can typically not be performed on all individual plants in each of the FMUs surveyed. Thus, they will be periodically measured during the cropping season, under ajn adopted sampling approach to keep in check the amount of work. For example, an FMU can be subdivided into square quadrats of 1 or 4 m2 area, from which a sample can be selected within which measurements will take place (Wagner et al., 2000). We adopted this approach in Mali/Nigeria. FMUs were sub-divided into 2 x 2 m quadrats, from which five were selected for measurement of seasonal canopy development variables. As much as possible, the quadrats selected must be evenly distributed over the FMU and represent the spatial variability (of the measured parameters) within the FMU (Figure 1).
On each visit (depending on the adopted frequency), it is advisable to take a series of geo-referenced pictures of the FMU to provide an overview of the field status and plant conditions. These pictures can be taken from multiple directions, i.e., outside or inside the field perimeter, before initiating field measurements.
Measurements in the selected quadrats should always follow a standard sequence (for instance, by clockwise direction) starting with the same quadrat. Within each quadrat, a horizontal picture can be taken at the centre before other measurement or biomass samples are taken.
Canopy development parameters that are worth monitoring during the cropping season are the following.
- Phenological development: this entails monitoring the different crop developmental stages, e.g. germination, leaf development, flowering, fruiting, maturity and senescence. For each FMU and each visit, one should register the developmental stage of the crop(s) cultivated. This information is critital to automatic crop identification routines on RS data.
- Plant height: The height of crop(s) should be measured at each visit. This information complements the phenological information above, as it depicts the continual growth of a crop to maturity. This can be used to validate canopy height models generated from UAV or other stereo images.
- Leaf Area Index (LAI): This index is the ratio of total leaf area (measured on one side of the leaf only) above a unit of ground area (m2/m2). The index characterizes plant canopy structure and is a metric for the biomass amount in the field. Crops may differ in LAI scores at various growth stages (i.e. due essentially to different leaf structure and size, and different speeds in plant development). Thus, with appropriate measurement equipment, one should measure the LAI of crops at each visit.
- Fraction of vegetation cover: This fraction is the percentage of soil surface covered by green vegetation. A natural, crop-specific relationship exists between this fCover fraction and LAI, and like LAI it varies with time. Measuring and monitoring crop ground cover can help to time properly interventions to reduce the susceptibility of the soil to erosion. Similar to LAI, this parameter usually differs between crops, and hence carries a potential for discriminating between crops. Another application of fCover is in irrigation scheduling.
- Chlorophyll content: This is the amount of chlorophyll of a plant per leaf area unit. Measuring can be done with a handheld device that works on the basis of light absorbance by the leaf measured. These measurements are relative, and normally one needs to use a monotonic function to convert into absolute chlorophyll values. The monitoring of different quadrats of a field over time may reveal variation in plant conditions (possibly signalling plant stress) across the FMU.