Impacts of Hydraulic Works on Climate and Hydrology
3.4. Impacts Caused by Construction of Hydraulic Works
The construction of hydraulic works generates physical changes within the watershed. For example, the construction of a dam on a river, with the formation of a reservoir, leads to impacts and effects on both climatic and hydrological aspects, as discussed below;
3.4.1. Climate Impact
At the macro level, the isolated effect of a dam on the regional climate may be negligible, because the main components of regional climate and its seasonal and annual variations are determined by the general movement of atmospheric circulation. At the local microclimate level, the effects are restricted to areas bordering the reservoir, causing small changes in climate variables such as temperature, relative humidity, rainfall, wind, and cloud cover.
a) Temperature
According to the results obtained for the reservoirs Jupiá-llha Solteira-Brasil (Tarifa, JR-1981), temperature seems to be the climate element that undergoes major changes due to the action of the reservoir, especially at the ends, where values tend to be flattened, i.e., a small reduction occurs in the daily temperature range, monthly, and yearly.
b) Relative Humidity (RH)
Analysis concerning the variation of relative humidity in the microregion of the Itaipu Dam (Brazil) (Tarifa, JR-1981) showed that the daily average values, with highs in the morning (95%) and lows in the afternoon (65%), measured in the period before the formation of the reservoir, were slightly higher than the values obtained after the formation of the reservoir. This is due to increased evaporation from the liquid surface, resulting in an increase in moisture in the boundary layer of air in contact with the water level of the reservoir and the effect of the wind. It is expected that the area surrounding the reservoir will present an increased number of days with dew, favoring the incubation and propagation of diseases and pests for crops and animals in the region.
c) Rainfall
Results of several studies show no change in the total values of annual rainfall after the formation of the reservoir. However, during dry and cold periods, there seems to have been a slight increase in mean rainfall, which can be explained by the possible formation of mist on cold mornings and increased moisture resulting in rainfall in the form of drizzle.
d) Wind
The change in surface roughness due to flooding causes changes in the vertical wind profile. With the decrease in surface roughness, there is a trend toward an increase in wind speed. Moreover, alterations in the balance of solar radiation and the action of weak winds induce a breeze mechanism. It is known that around a large lake, the wind blows toward land during the day (lake breeze) and toward the lake at night (land breeze). When the wind starts blowing from the lake, the temperature drops, humidity rises, and the wind moves over the land like a cold front. As Yoshino (1975) noted, between the area controlled by the lake breeze and adjacent areas free of this effect, the temperature difference can reach up to 4 °C. The significant effect of this situation is the possible formation of ripples on the lake, and the presence of a temperature change can be uncomfortable for the coastal population.
e) Cloudiness
Mists, a product of evaporation, are more intense during the winter when cold air moves over the warmer liquid surface. Moisture from the water by evaporation, coupled with cold air, causes condensation to become saturated. These mists are concentrated in the lower parts (valleys) and, of course, the sector most likely to be affected is the one that sits in the prevailing wind direction.
Weather
a) Type or Form of Precipitation
The type or form of precipitation is of great importance in the variation of river flow. For example, precipitation as rain, with intensity and extent sufficient to influence runoff, is felt almost immediately, while precipitation as snow, without reaching the melting temperature, will not be felt.
b) Intensity of Rain
When rainfall intensity exceeds the soil infiltration rate, surface runoff occurs due to excess rainfall. The higher the rainfall intensity, the higher the stream flow. It can be concluded that after the infiltration capacity is exceeded, surface runoff will increase rapidly with increasing rainfall intensity. However, the increased river flow is not proportional to the increase in heavy rain because of the delay effect resulting from the accumulation process.
c) Duration of Precipitation
Rainfall with a duration below the time of concentration of the basin, regardless of intensity, has virtually the same period of surface runoff, while for long rains, the runoff period will be higher. Another effect of the duration of precipitation is that the infiltration capacity decreases during the rain.
d) Distribution of Precipitation in the Basin
The uniform distribution of precipitation on a watershed happens rarely. For small basins, peak flows occur for high-intensity rainfall that covers small areas, while for large basins, peak flows occur for low-intensity rainfall that covers large areas. For example, in Figures 3.7A and 3.7B, curves are presented with the same high rainfall (isohyets) of two showers. Assuming that the highest rainfall totals are nearly equal, the resulting hydrographs can be very different. In the case of Figure 3.7A, there may have occurred little or no surface runoff, depending on the soil infiltration capacity. In the case of Figure 3.7B, probably in the lower section, the infiltration capacity was greatly exceeded.
e) Direction of Rain
The direction that the rain moves through the watershed in relation to the direction of flow of the drainage system has a great influence on the resulting peak flow and duration of surface runoff. Assuming that the rainfall intensities 1, 2, and 3, as shown in Figure 3.8, are equal, the hydrographs resulting from the rains at the gauging station will be very different.
f) Precipitation and Previous Soil Moisture
The moisture content of soil layers influences the infiltration capacity and also determines the possibility of increasing soil water. When soil moisture is high, the infiltration capacity is low, and the drainage basin is susceptible to flooding. On the other hand, when the soil moisture content reaches the retention capacity of the soil, seepage water will reach the water table. A rain that falls shortly after a previous one can cause considerable peak flow, while the same rain after a dry period will not produce significant flows. An area of virgin forest with a thick layer of leaf litter, twigs, and grass can withstand high rainfall without runoff; however, the same area, where it has been harvested and processed into a town center, can be compacted by the traffic of people and animals, resulting in surface runoff that can cause flooding. Conversely, an open field replaced by dense plant cover can lead to increased infiltration capacity and reduced surface runoff.