1. Background of The Model
2. GenRiver Overview
3. GenRiver Components
Concerns over negative environmental effects of forest conversion are often expressed in relation to hydrological impacts. Patterns of river flow are widely perceived to change upon change in land cover from closed forest cover to an agriculturally used landscape. Different aspects of river flow such as annual water yield, the partitioning over storm flow and base flow, and changes in water quality may, however, occur at different rates, Change to different degrees, and may even move in different direction (Calder, 2002). Various approaches exist for modeling watershed functions, ranging from directly data driven (empirical) approaches to models based on concepts of a water balance, soil physics and hydrology. Hydrology models differ by temporal and spatial scale. A detailed level model use with detailed description of rainfall and infiltration may require a minute (or even seconds) time step, especially on slopes where water will become surface runoff if it cannot infiltrate within seconds of reaching the soil surface. At the other end of the spectrum we may find empirical equations relating annual water yield of a catchment to annual rainfall (or precipitation in climate zones where snowfall and ice rains are significant).
GenRiver is a distributed process-based model that extends a plot-level water balance to subcatchment level. It was developed for data -scarce situations and is based on empirical equations. The model can be used to explore the basic changes of river flow characteristics across spatial scales - from patch level, subcatchment to catchment. GenRiver is simple river flow model, can be use as a tool to explore our understanding of historical changes in river flow due to land use change.
The model treats a river as a summation of streams, each originating in a subcatchment with its own daily rainfall, yearly landcover fractions and constant total area and distance to the river outflow (or measurement) point. Interactions between streams in their contribution to the river are considered to be negligible (i.e. there is no 'backflow' problem). Spatial patterns in daily rainfall events are translated into average daily rainfall in each subcatchment in a separate module. The subcatchment model represents interception, infiltration into soil, rapid percolation into subsoil, surface flow of water and rapid lateral subsurface flow into streams with parameters that can vary between land cover classes.
Land cover change can significantly affect watershed functions through changes in the fraction of rainfall that reaches the ground, the subsequent pathways of water flow over and through the soil and the rate of water use by plants. Simple characteristics of the vegetation (monthly pattern of leaf biomass, influencing canopy interception and transpiration, and ability to extracts water from deeper soil layers) and soil (especially compaction of the macropores in the soil that store water between ‘saturation' and ‘field capacity') can probably explain a major part of the impacts on river flow.
Empirical assessment of the dynamics of water flows as a function of land cover change and soil properties takes time and resources, and needs to take temporal and spatial variation of rainfall into account. A model based on ‘first principles' that integrates land cover change and change in soil properties as driving factors of changes in river flow can be used as a tool to explore scenario's of land use change, if it passes a ‘validation' test against observed data.
GenRiver is a generic river model on river flow. As is common in hydrology, it start the accounting with rainfall or precipitation (P) and traces the subsequent flows and storage in the landscape, that can lead to either evapotranspiration (E), river flow (Q) or change in storage (ΔS) (Figure 1):
P = Q + E + ΔS [1]
Models differ in the relations between the different terms of the balance equation and in the way they account for the ‘slow flows', that derive from water that infiltrates into the soil but can take a range of pathways, with various residence times, to reach the streams and rivers, depending on land form, geology and extractions along the way.
The core of the GenRiver model is a ‘patch' level representation of a daily water balance, driven by local rainfall and modified by the land cover and land cover change and soil properties of the patch. The patch can contribute to three types of stream flow: surface-quick flow on the day of the rainfall event, soil-quick flow on the next day and base flow, via the gradual release of groundwater.
A river is treated as a summation of streams, each originating in a subcatchment with its own daily rainfall, yearly land cover fractions and constant total area and distance to the river outflow (or measurement) point. Interactions between streams in their contribution to the river are considered to be negligible (i.e. there is no ‘backflow' problem). Spatial patterns in daily rainfall events are translated into average daily rainfall in each subcatchment. The subcatchment model represents interception, infiltration into soil, rapid percolation into subsoil, surface flow of water and rapid lateral subsurface flow into streams with parameters that can vary between land cover classes.
Figure 1. Overview of the GenRiver model; the multiple subcatchments that make up the catchment as a whole can differ in basic soil properties, land cover fractions that affect interception, soil structure (infiltration rate) and seasonal pattern of water use by the vegetation. The subcatchment will also typically differ in ‘routing time' or in the time it takes the streams and river to reach the observation point of main interest
GenRiver model consists of several sectors, which are
related to one another. Those sectors are:
- Water Balance is a main sector
that calculating the input, output, and storage changes of water in the
systems. Some components which are in this sector, rainfall, interception,
infiltration, percolation, soil water, surface flow, soil discharge, deep
infiltration, ground water area and base flow - Stream Network is a sector that
estimating the flow of water from the river to the final outlet. Some
components which are in this sector, total ttream in flow, routing time,
direct surface flow, delay surface flow, river flow to final outlet. - Land Cover
is a sector to generate land cover data per sub
catchment for each year, - Subcatachment Parameter is a sector stired constant parameters that control to the changes of water balance, landcover and stream network.
GenRiver
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