Crop development information
Surveys in this category focus on canopy development or growth of the crop(s) cultivated on the FMU throughout the cropping season. The frequency at which such information should be collected during the cropping season will depend, among other things, on the crop type, the parameter being measured, available labor vis-à-vis measurement equipment and the frequency of satellite image acquisition. Within the STARS project, information on crop canopy development were collected every 2 weeks.
Because these parameters will be periodically measured during the cropping season, a sampling approach needs to be adopted in order to reduce the amount of work. For example, a FMU can be sub-divided 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). This approach was, for instance, adopted by the STARS team 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 should be evenly distributed over the FMU and represent the spatial variability within it (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 snapshot of the general 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 (e.g. clockwise direction) starting with quadrat 1, quadrat 2, quadrat 3, quadrat 4, quadrat 5, etc. Within each quadrat, a horizontal picture can be taken at the centre before any other measurement or biomass samples are taken.
Canopy development parameters that are worth monitoring during the cropping season are:
- 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, it is important to note the developmental stage of the crop(s) cultivated. This information is fundamental to crop identification on RS data.
- Plant height: The height of crop(s) should be measured at each visit to an FMU. This information complements the phenological information in (1) above, as it depicts the continual growth of a crop up to maturity. This can be used to, among other things, validate canopy height models generated from UAV or other stereo images.
- Leaf Area Index (LAI): LAI is defined as the ratio of one sided leaf area per unit ground area (m2/m2). The index characterizes plant canopy structure and gives an idea of the amount of biomass available in a field. Different crop types may have different LAI and at different growth stages (i.e. due essentially to different leaf structure/sizes). Thus, if the appropriate measurement equipment is available, it is important to measure the LAI of crops at each visit.
- Ground crop cover/fraction: this is defined as the percentage of plant material which covers the soil surface. There is a natural relationship between this and LAI. Measuring and monitoring crop ground cover can assist in reducing the susceptibility of soils to erosion. Similar to LAI, this parameter could differ for different crops, and hence potential for separability during crop classification. Other application of this information is in irrigation scheduling.
- Chlorophyll content: this indicates the level/amount of greenness of a plant. Measuring and monitoring this in different quadrats of a field over time may reveal variation in plant conditions (e.g. plant stress) across a FMU.