Our research project is looking to do a classification of vegetation in a dynamic environment. The environment is an annually flooded floodplain and the dominant vegetation types are annual and perennial grasses. The aim is to create distribution maps of the different grasses at different temporal points during the flood cycle. We have some ground-truthed transect data but would like to extrapolate that to the whole floodplain. The goal is to describe changes in the quality and quantity of vegetation over the years for the whole floodplain and link these to changes in the flooding patters.
We are looking for any guidance on: (i) the methodological approach: (ii) the remote sensing data (we are currently using Landsat); and (iii) how to deal with the potential dynamics in the relationship between the annual growth cycle and spectral signature of the grasses in the floodplain.
The first comment has much merit. Lidar is such a great tool to address surface differences, refine soil maps, and perhaps help identify flooding patterns. If you have nearby long term stream gauging stations, and can relate the flooding frequency of those to your area, you might have a tool to project flooding frequency to your area of interest. Some plants do well with flooding, others not, some aggressive early successional responders. In your data collections, you might be able to evaluate if you have vegetation systems that invade or just respond after flooding, a big difference I would think. The soil types and whether they are well drained, or at other extreme hydric may make some differences. Data from the soil maps may or may not be helpful, depending on how recent and how accurate. Many floodplains are not mapped that specifically for soils and these floodplains or sometimes riparian terrace areas can contain a variety of colluvial, fluvial and alluvial processes in depositing and reworking sediments. Knowing your stream and valley type classification (Dave Rosgen - wildlandhydrology.com various free references) can be helpful in understanding if you are in a wet meadow E type stream, a braided stream D type, C type stream or G, F or maybe a few B stream sections within a valley with no or limited floodplain access, and how valley morphology developed may be helpful background in understanding the flooding and sedimentation regime. You mentioned annual flooding, and that suggests you may have a flooding frequency perhaps higher than normal such as for an aggrading system where the bankfull flow is reached every year or perhaps snowmelt or ice in channel flooding, and the channel capacity is exceeded due to ice, or other reasons. Some channels adjust their morphology enough that flooding occurs in most years, but not every year. There is a lot of potential with spectral imagery and the potential applications that I am not an expert on.
Hi Mark. You do not indicate under which climate is the floodplain you are working with. My experience is from the monsoonal tropics, in the inner delta of the Niger river (in Mali). A high resolution mapping of the vegetation composed of wetlands, flooded savannahs, but also steppes and thickets on levees was done from aerial photos and field work. Then a number of sites were monitored over seasons and years to document and understand the dynamics. Up scaling these site data to the whole flood plain using satellite remote sensing is challenge. NDVI for example is affected by water background in flooded areas, by the turbidity of the water, even by soil moisture/dry soil, and tends to saturate at low green vegetation cover in these conditions (see Hanan et al. Int J of R S 1991). However, remote sensing is useful to monitor the progression of the flood, providing high spatial and temporal resolution. Then for each vegetation type, modelling can help the estimation of production based on rainfall and flood regime documented by remote sensing. The recent work by Crystele Leauthaud on the production of the Echinochloa stagnina dominated wetland in the Tana River delta (Kenya) provides a good example of application.(her thesis in 2013 in Montpellier title
ON THE INFLUENCE OF FLOODS ON ECOSYSTEM SERVICES
OF FLOODPLAIN GRASSLANDS
An application to fodder production in the Tana River Delta, Kenya
She as published a couple of papers...
Good luck,
Pierre
You may find some useful hints in the attached from the Kafue floodplain in Zambia.
Chapter Impact of shrub encroachment on the understory vegetation di...
A prof of the National School of Engineees in Bamako Mali has worked on separating inundated vegetation from soil vegetation from NOAA data over the surface of the Niger River Inner Delta in Mali.
It can help.
Conference Paper Estimation of the flooded area of the Inner Delta of the Riv...
Article Les surfaces inondées dans le delta intérieur du Niger au Ma...
Article Monitoring flood propagation in the Niger River Inner Delta ...
The area may not be wetland, but you might look into the wetland mapping by US FWS conducted using Cowardin et al classification, soil series maps of floodplain area may or may not be helpful depending on the accuracy. Is flooding the only disturbance, or has farming, burning or other activities removed and kept the bottomland trees out? Are rushes and sedges also present as they are indicators of wetness. Also water table detail such as from well log, or at least normal water table as indicated by the hydric soil indicators. The USFS has an inventory method used for groundwater dependent ecosystems that may also apply if looking into methods of sampling, describing, etc. Is the floodplain cover type natural for conditions, or maintained in grassy condition for habitat or other benefit? These are some of the questions you may want to consider. Floodplains are not necessarily uniform, as sediment type, wetness, properties can vary across the floodplain, and may explain some of the vegetation differences, so collaborating with a soil scientist may help.
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