Estimating the Water Requirements for Plants of Floodplain Wetlands

flood regime. These categories were based on visual inspection andexpert interpretation of the flood regime for the ‘natural’ scenario fromIQQM. The natural scenario uses the historic rainfall series to generateflows in the river system with no regulation and no diversions. Onehundred-yearsimulations using IQQM therefore allowed time series ofthe health status of each vegetation class to be generated.Category 2: hydrologic/empiricalApproaches in this category again rely on hydrologic modelling (waterbalance) of the water regime, but use empirical vegetation–hydrologyrelationships derived from analysis of relevant data. The empiricalrelationships may have been derived for the location being investigated,or may have been derived from prior investigations in similar locations.The empirical relationships may be based primarily on analysis of spatialdata, or on analysis of time series data, or on complete analysis of spatiotemporalpatterns. The nature of the available relationships will dictatethe nature of the possible predictions. Relationships may take variousforms, including regression models (linear or higher orderrelationships), and probabilistic models based on definition of jointprobability distributions; for example, the probability of vegetationmeasure ‘A’ occurring in water regime class ‘B’ (Section 4).Prairie wetlands, ND. An example is the modelling work of Poiani andJohnson (1993) for two semi-permanent prairie wetlands in NorthDakota. The modelling included a simple daily water balance model forindividual wetlands. The water balance included direct rainfall inputs,local run-off, and evapotranspiration. Groundwater exchanges were notdirectly considered. The water elevations determined by the waterbalance model were used together with ground elevation data tocalculate water depths for each cell in a grid representation of thewetland. Six vegetation types (of species with similar life-histories) andan open water category were used in the vegetation response model.The vegetation response model is a series of rules that describes theseasonal water regime conditions necessary to effect a change from onevegetation category to another. These rules were based on data,observations and analysis from a significant amount of prior fieldresearch by various investigators in these prairie wetlands. Both modelswere developed and calibrated using data from one wetland, and thenrun and evaluated for a second wetland.Riparian wetlands, Ontario. A second example is the logisticregression vegetation–hydrology model of Toner and Keddy (1997) forriparian wetlands in Ontario, Canada. In this investigation, models weredeveloped to simply predict the presence or absence of woody cover, asdetermined by locating the woody–herbaceous boundary within awetland. A set of seven hydrologic variables was selected to reflect thedepth, duration and time of flooding. Relationships between vegetation(woody or herbaceous) and the hydrologic variables were establishedby logistic regressions. Relationships were developed using first, eachhydrology variable and four other site variables, and second, all possiblecombinations of the seven hydrologic variables. Statistical criteria wereused to select the best model generated. The model was notindependently validated by the authors, nor was it linked to a hydrologymodel for use in scenario simulations.Section 7: Predicting Vegetation Responses 87