Configuración de un conflicto ambiental minero: 2010 - 2014

Here we take a simple worked example to illustrate how runoff efficiency can be calculated. Assume a river basin area of 80 km2_{, precipitation of 1600 mm in a year and that there was a river gauging} station which told us that there had been 72 million cubic metres of water discharge in total during the year. What we need to do is put everything into the same units. One way of doing this is to put everything into units of length (depth of water, just like rainfall). So imagine that you spread all of the 72 million cubic metres of water across the river basin evenly; then you would have a depth of water equivalent to: 72 000 000 m3_{divided by the area of the basin, which is 80 000 000 m}2_{, which gives us} 0.9 m or 900 mm. If we compare this depth of water produced as river discharge from the basin to the precipitation that fell on the basin (1600 mm), then we can see that this equates to an efficiency of 56.25%. In other words, 0.5625 of the precipitation that fell on the basin was released from the river basin outlet as river discharge. The remaining 43.75% of water was either lost from the catchment as evapotranspiration or resulted in a change of storage in lakes or groundwater. Similar calculations can be performed for individual storm events.

BOX 3.1 TECHNIQUES

REFLECTIVE QUESTION

What are the main processes of water movement through and across the landscape to reach rivers and lakes?

C RIVER FLOW

1 Hydrographs and river regimes

Rivers are supplied by throughflow (including groundwater) and overland flow. The relative proportions of the different types of flow, along with the soil types, topography, size and type of drainage network, can determine how quickly river flow varies during rainfall events or season - ally. River flows can change during individual rainfall or snowmelt events, or remain fairly stable, depending on the nature of the river basin. There is a delay (lag) between the precipitation occurring and the peak discharge of a river.

Where infiltration-excess overland flow dom - inates the runoff response, then the hydrograph (graph showing discharge through time) is likely to have a short lag time and high peak flow (Figure 3.7a). The storm hydrograph showing river dis - charge through time in response to a rainfall or melt event will therefore be quite steep in shape, rising quickly from low flow to the peak flow. Urbanisation increases flood risk, as it reduces the infiltration capacity of the surface through construction, leading to rapid water flow to the river and resulting in steeply rising hydrographs with sharp peaks. If matrix throughflow domin - ates flow pathways within the river basin, then the river may rise and fall very slowly in response to precipitation and the peak may be small. How - ever, since throughflow contributes to saturation- excess overland flow, then throughflow can in many circumstances still lead to rapid and large flood peaks. Even without saturation-excess overland flow, a throughflow-dominated catch - ment may still produce storm discharge if water can get through the soils quickly (e.g. if the soils are very permeable). In some soils, such as peat, only a small amount of infiltration may be needed to cause the water table to rise to the surface (Evans et al., 1999). In other soils there may even be two river discharge peaks caused by one rain- fall event (Figure 3.7b). This might occur where the first peak is saturation-excess overland flow

dom inated, or perhaps related to the direct deliv - ery of rainfall into the river channel, and the second peak a little later may be much longer and larger and caused by subsurface throughflow accum ulating at the bottom of hillslopes and in valley bottoms before entering the stream channel (Anderson and Burt, 1978). Throughflow may also contribute directly to storm hydrographs by a mechanism called piston or displacement flow. This is where soil water at the bottom of a slope is rapidly pushed out of the soil by new, freshly infiltrating water entering at the top of a slope.

Figure 3.8shows flows in the form of hydro - graphs over one year for two nearby rivers where the climate is the same. Despite being in the same area, the flows are very different between the

Figure 3.7 Storm hydrographs (a) where infiltration-excess overland flow dominates and (b) where throughflow dominates. Note that (a) also indicates typical hydrograph terminology.

rivers. The flows in River 1 appear to be domin - ated by baseflow and there are no individual storm peaks, unlike for River 2, which appears to be more dominated by overland flow or rapid throughflow (e.g. via macropores). The River 1 basin overlies permeable bedrock, is gently slop - ing, and has soils that enable good infiltration and little chance of saturation to the surface. There - fore, infiltration-excess or saturation-excess over - land flows are a rare occurrence here. For River 2, however, the soils are thin and sit above imperm - eable bedrock and so there is frequent saturation of the soils and generation of saturation-excess overland flow.

Seasonal variability in river flow is known as the river regime, for which there are four major global types:

1 Arid zones, especially in subtropical drylands, tend to experience very occasional but intense rainfall events. Intense rainfall along with little vegetation cover produces infiltration-excess overland flow, rapid runoff and high flood peaks. However, many dryland soils are coarse and sandy, with high infiltration capacities, resulting in little chance of overland flow. Therefore, there is a wide variation of response even if rainfall intensities are very high. In most drylands river flow will stop within a few days

of the rainstorm and water often seeps into the river beds or is evaporated. River flow is therefore highly intermittent in these systems. 2 Where snow and ice melt dominate, then there can be a major peak of river flow during the late spring or early summer (e.g. Danube in Hungary, or Mackenzie in North America). River discharge can be extremely low during the winter months on some rivers, even though precipitation may be continuing, as this pre - cipitation is stored on glaciers or snowpacks in winter. There can also be a strong daily change in river discharge due to daily melt cycles of the snow and ice. Night-time discharge immedi - ately downstream from glaciers tends to be much lower than that of the mid-afternoon. 3 In temperate, oceanic areas precipitation

occurs all year, perhaps with seasonal max - imums. The river flow regime in these areas can change in response to either seasonal changes in groundwater storage and release, or higher evaporation and transpiration rates during the summer months (e.g. Seine, France).

4 Rivers in equatorial areas tend to have a fairly regular regime, while tropical river systems outside of the equatorial areas receive high precipitation during the summer but experi - ence a marked dry season during the winter. Evaporation and transpiration are high at all times so that the streamflow mirrors the seasonal pattern of rainfall. Rivers such as the Brahmaputra or Mekong in south Asia have summer peaks associated with monsoon rains.

2 Closed basins

While most rivers around the world drain into the oceans (exorheic basins), there are many that do not, and these are often termed ‘closed basins’ or endorheic basins. Almost one-fifth of the Earth’s land surface does not drain to an ocean but instead drains to an inland ‘sink’ (Figure 3.9). These tend to be in the interior of continents, in dryland regions, or where surrounding topo-

Figure 3.8 Two example annual river flow records showing regimes of each river (after Holden, 2011).

g raphy such as a mountain range prevents water from reaching the ocean. The largest of these endorheic basins covers much of continental Asia and river water here can end up in the Caspian Sea and Aral Sea. The Great Basin (which is actually several adjacent closed basins) in the USA and much of central Australia (e.g. Lake Eyre basin) are other examples. The water in lakes into which endorheic rivers drain tends to be very saline because evaporation is a major water loss, leaving behind concentrated salts. For example, the Dead Sea, at the end of the River Jordan, which is located at the lowest point on the surface of the Earth, is so saline that people can lie in the water without sinking. In the centre of many endorheic basins there is no permanent standing water body, but there can often be a white ‘salt pan’ where once a lake existed, or where the lake occurs only seasonally.

3 Measuring river discharge

Good measurement of river discharge is important for water resource management, flood warnings and monitoring for impacts of environmental change. Rivers act as a point of concentrated water movement where the confinement of water to a channel can assist in measuring the discharge. Typically, gauging stations consist of a method to measure the water levels (such as a pressure sensor) and a method to convert water levels into discharge. To help with this process many gauging sites use weirs or flumes, although these are not always practical (e.g. on large rivers). The weirs and flumes provide more control over the relationship between water level and discharge. This relationship is known as a rating equation or a stage-discharge equation. Sometimes a design of weir has a standard engineering rating equation, but even if this is the case, testing in the field is recommended. To

derive a stage-discharge equation, so that from any water level at the point in the river being studied we can calculate the discharge, requires us to measure the discharge of the river at a range of water levels.

Various techniques can be used to measure river discharge, including dilution gauging, whereby a known volume and concentration of a tracer, such as salt solution, is added to the river. The more water in the river, the more diluted the salt solution will become. There - fore, measuring the salt’s concentration slightly downstream enables us to work out the dilu - tion of the salt and hence what the river discharge is. For a more detailed explanation of salt dilu- tion gauging see the short video at http://tinyurl. com/aopjqxb. Alternatively, the cross-sectional area of the river can be measured and then the river cross-section is split into different segments and the velocity of the water passing each segment is measured. The water velocity may be measured using an impeller meter (typically at 0.6 of the depth as an assumed average of the water velocity in the entire water column; water tends to be slower near the bed, due to friction, and faster near the river surface) or other device (often done by someone standing in the water hold ing the device). Multiplying the

velocity (m s–1_{) by the area of the segment (m}2₎

produces the discharge (m3_s–1_{) for that seg ment}

of the cross-section (Figure 3.10). Sum ming across all segments in the cross-section then yields the total river discharge. This sort of technique can be impractical at very high river flows or where the river is highly vege tated, has lots of boulders or has several shallow channels.

Ultrasonic discharge gauges record the time for acoustic pulses to cross a river at different depths. Devices are placed on opposite banks of the river and the travel time for the pulse is used to calculate the streamflow velocity. A water level sensor is also needed and the cross- sectional area has to be surveyed in advance for different water levels. Doppler probes record the change in frequency when acoustic waves are reflected from natural suspended sediment or gas bubbles in river water. The change in frequency is proportional to the velocity of the particle or gas bubble. The Doppler method therefore works well in murky rivers, unlike ultrasonic discharge gauges described above. Sometimes Doppler profilers are deployed from boats or from cables across rivers to provide velocity measurements across the channel. Measuring river discharge can be very costly and is impractical in many very remote places or for large rivers. In fact the

number of gauging stations is reducing worldwide due to cost (Vörösmarty et al., 2000). However, work is ongoing to refine the use of satellite data to determine river discharge, as described in Box 3.2.

4 Models of river discharge

Models that can predict how rainfall (either real or simulated) produces river discharge can be very useful for water resource planning, flood forecasting, designing flood defence systems and in estimating river flow in basins that have no river gauges. They can also help us understand how river basin systems work, and we can perform landscape-scale experiments using models that we might not be able to do in the real world. Predicting river discharge from rainfall is not as simple as it might at first sound, particularly if the basin is large, because that will most likely mean that the basin has many different types of landscape within it. The river discharge that results from rainfall will vary with antecedent conditions because these affect infiltration rates and saturation conditions, and will vary with the distribution of rainfall across the basin through time. Many hundreds of models have been devel - oped that are used to predict river discharge, and these vary in complexity. However, the quality of data input into models is an important con - sideration that will affect how good the pre dic- tion of discharge is. All models, no matter how com plex, are just approximations of reality, and so the choice of model to be used depends on the questions being asked and the type of river basin being studied.

One of the simplest discharge models is known as the unit hydrograph, and was developed by Sherman (1932, 1942). The unit hydrograph is the stormflow that would be produced from an effective rainfall (that amount of rainfall which is released by the river as stormflow) over a unit of time (such as one hour or one day). The model assumes that the rainfall has occurred uniformly through time and across the basin. In essence, the

unit hydrograph model requires users to obtain a storm hydrograph, work out the total volume of stormflow (in units of millimetres; i.e. average depth of water if spread across the catchment), and call that the effective rainfall. Then users should divide the hydrograph size (i.e. the y axis) by the effective rainfall so that what we are left with is the size and shape of hydrograph produced by 1 mm of effective rainfall. The hydrograph will now have units of m3_s–1_{per mm of rain. This is}

now the unit hydrograph for the river basin. In order to predict the river discharge for new rainfall events (either real ones or ones that we have made up to test, for example, how big a flood might be) we need to establish how much of the rainfall is effective rainfall. This can be done by using the first storm we derived the unit hydrograph from by subtracting effective rainfall from actual rainfall