Capital y Ciudadanía: limitaciones materiales a una ciudadanía efectiva

The Surface Water Ocean Topography (SWOT) mission, supported by NASA and Centre National d’Etudes Spatiales, seeks to directly measure water heights from space with an accuracy of a few centimetres. The mission plans to measure water levels every 100 m across the entire globe every 10 days. Such a dataset would have the potential to hugely improve our understanding of the global water cycle and to monitor river and lake levels. Additionally, by measuring the topography of the river at low- and high-water levels from space, cross-sectional areas can be derived for different water levels. Combining water level data with cross- sectional information and the slope of the river, all derived from space, could enable river discharge to be estimated for rivers across the globe at 10-day intervals. For further information please see http:// swot.jpl.nasa.gov.

and averaging the remainder over time to give us an average ‘infiltration rate’ in mm hr–1_{. This}

infiltration rate is really just the average amount of rainfall that does not produce storm discharge and is not a real infiltration rate for the soils in the river basin. It is often called a phi-index. If we use that same phi-index for future rainfall events, we can then determine how much of the rainfall is effective rainfall. If we are then left with, say, 1 mm, 3 mm, 2 mm of effective rainfall over three consecutive hours, then we can establish the pre - dicted hydrograph by multiplying the unit hydro - graph by one in hour one and then making a new hydrograph starting one hour later which is three times the size of the unit hydrograph and finally, in the third hour, we have a hydrograph which is twice the unit hydrograph (Figure 3.11). The next step is simply to add the three storm hydrographs together to get overall predicted storm discharge from the rainfall event. If we wanted to predict overall river discharge, then the final step is to add baseflow.

Clearly, the unit hydrograph model is simple to use, but it also entails a lot of assumptions about uniformity in the river basin and does not deal with spatial processes such as the variable source area concept and issues around antecedent conditions. Models such as the unit hydrograph suffer when tested for different conditions because the same amount of rainfall, even if uniformly distributed across the river basin, can produce a very different storm discharge depending on ante - cedent conditions (e.g. wet soils or dry soils). There has therefore been development of a suite of models, such as Topmodel and SHE, that try to capture elements of spatial flow processes and saturation processes. Basic overviews of those and other models are provided in Shaw et al. (2011), with more detail in Beven (2001), but in essence such models use the topography of the river basin and some information about soil types and satura - tion conditions to move water through and across the landscape to the river system.

River flow models are typically tested by com paring a predicted river discharge with the

observed river discharge. After some fine tuning to optimise the model to work best (it will never be perfect), then it can be applied to situations where we do not yet have river discharge data (e.g. future events, simulated storm rainfall, a change in land management (e.g. see Box 3.3), ungauged river basins). However, users of such models must always be aware that the predictions are just approximate estimates and it is useful for any predictions to be presented along with an estimate of how far we think the prediction might be wrong.

Figure 3.11 Predicted river storm flow discharge using the unit hydrograph model. Each hour of rain- fall is associated with its own hydrograph. The first hy drograph produced from the first pulse of rainfall results from an effective rainfall of 1 mm and is there - fore the basic unit hydrograph. The second hydro - graph results from the second pulse of effective rainfall, which is 3 mm, and the hydrograph is there - fore three times the size of the first one. All of the smaller hydrographs resulting from each pulse of effective rainfall are added together to produce the overall storm hydrograph for the catchment predicted to result from the rainfall event shown. Baseflow is not shown but if you wanted to predict total river dis - charge you would simply add baseflow to total stormflow.

5 Impacts of land management on river flow

Land management change can dramatically alter the flows in river systems. Building dams and changing or diverting stream courses alters river regimes. In fact the Colorado River, which is the main river in the southwest of the USA, has been so heavily modified through major dams and abstraction schemes that discharge along its lower course is massively reduced so that it now rarely flows to its natural exit in the Gulf of California.

Major land management change may alter the water budgets of river basins. For example, deforestation may result in a large reduction in transpiration rates. In 1934 a hydrological mon itor ing station was set up at Coweeta in the southern Appalachian Mountains to look at how forest management affected river flows. The research showed that the amount of water flow- ing from a forested basin increases after timber harvest, mainly because of reduced transpiration from trees. The volume of water from storms (efficiency of the river basin) and peak flow increases after deforestation. As the vegetation grows back and trees begin to recover, the peak flows and the basin efficiency start to decline again.

Any management activity that alters the flow - paths for water and the proportion of water moving via that flowpath – infiltration/saturation- excess overland flow, matrix or macropore flow – has the potential to alter the river hydrograph responses to storm events or the regime of the river. Agricultural activity can have a major im - pact on soil infiltration. Ploughing can often increase the infiltration capacity of the soil, which may then decline during the cropping season until the soil is ploughed once more (e.g. Imeson and Kwaad, 1990). It has been shown that livestock grazing has profoundly affected soils throughout the world, from arid and semi-arid rangelands to temperate moorlands. A review by Greenwood and McKenzie (2001) suggested that grazing ani - mals exert pressure on the ground comparable to

that of agricultural machinery, which leads to soil compaction. Research has generally shown that as grazing pressure increases, vegetation cover declines, soil bulk density increases, water infil - tration rates decrease and surface runoff levels increase (Abdel-Magid et al., 1987; Mwendera and Saleem, 1997). Evidence from a UK study showed that alterations of vegetation and soil properties due to grazing greatly increased active source areas for overland flow and discharge during storm events (Meyles et al., 2006).

Field under-drainage and the use of open ditches have been common land management techniques in agricultural areas. These systems would tend to be installed where the land is deemed to be too wet for optimum agricultural performance and their aim is to lower the water table. Such drainage often contributes to river regime by increasing baseflows and potentially increasing the flood peak. Drains and ditches act as preferential flowpaths for the fast channelling of water to watercourses. In peatland systems the use of open ditches has lowered the water table and caused organic soils to dry and crack, which has resulted in enhanced flow of water through macropores and the expansion of natural pipe networks (Holden, 2005b). Flow through these pipes can erode the system from below, deliver a lot of carbon to the stream (Holden et al., 2012) and lead to gully development when pipes collapse.

Urban development often reduces the local infiltration capacity to zero, which can increase the propensity for downstream flooding (Perry and Nawaz, 2008). The loss of vegetation cover also reduces transpiration and hence increases the annual and storm efficiency of the river basin. Urban areas tend to have a subsurface drainage system which takes water away from the surface using sinks which pipe it to water courses. Tradi - tionally these underground drainage systems have served as fast pathways for water movement across the basin and have increased flood peaks. More modern developments use sustainable urban drain age systems, which try to minimise

the downstream impact on river flows of the urban development (see Chapter 4, section F2.2). They often do this by encouraging the use of more permeable surfaces and also by having pipes and conduits that discharge into a series of ponds or storm water-collecting channels which then drain more slowly into the river network at a later stage. In essence, sustainable urban drainage systems have made space for storm water in their design. However, during intense rainfall events these systems can still be overwhelmed; clearly, the costs and space required for large storage are greater and so a judgement has to be made as to how much space to allocate for sustainable urban drain age, without overly compromising the aes - thetics of the landscape or the commercial value of the land. Many sustainable urban drain age systems also seek to clean the water before return - ing it to rivers (e.g. by using lake or wetland

systems to settle out sediment). These systems can be designed to look aesthetically pleasing and perhaps also serve other purposes such as pro - viding recreational and leisure services (Figure 3.12).

Predicting the effects of land management change on river flow is not straightforward because river basins are not uniform and have varying soils, topography and drainage networks. Therefore river basin models are often deployed to investigate the potential impacts on river flow of management interventions (e.g. Box 3.3). In fact the same land management can have a very different impact on river discharge (e.g. the flood peak), depending on where in the river basin that management takes place. Section D provides further explanation of this issue. These spatial effects, related to how water is routed through the drainage basin and the timing of delivery of water

Figure 3.12 New housing development in Calgary with a sustainable urban drainage system with a storm water storage lake that creates an aesthetic scene for the houses.

Modelling land use change impacts on river discharge: