WaterSubject

Evapotranspiration refers to the combined effect of evaporation and the use (and subsequent expiration) of water by vegetation. Infiltration usually describes water passing into the groundwater, and therefore may be regarded as a loss or outflow. However, it is more correctly considered as a form of storage, either in surface depressions or within the surface of paving materials. The term Storage can refer to groundwater storage as well as depression storage on the surface and temporary interception by trees and vegetation. In the long term, the change in storage can usually be neglected.The water balance concept can also be applied to urban areas. However, the complexity of the urban water cycle can make its application on a catchment scale more difficult On a smaller scale, the difference between precipitation and runoff for paved surfaces is largely accounted for by losses due to surface wetting and detention as well as infiltration through joints and cracks. In such cases, storage would include water that is held in the top few centimetres of the ground surface.

Evaporation and infiltration on paved surfaces depend on a dynamic interaction between the microclimate above the surface (particularly temperature, wind speed and relative humidity) and the distribution of moisture within and on the surface. The moisture distribution itself is a function of the material properties, the surface texture and the microtopography of the surface in terms of surface depressions that lead to detention storage. Although evaporation losses during storm conditions are minor and are often neglected, over the long term evapotranspiration is often the largest output in the urban water balance. In one study, evaporation from roof surfaces accounted for 19% of rainfall, and infiltration through road surfaces accounted for 36% of rainfall (more than twice the runoff).

In order to investigate the water balance of various paved surfaces, a series of tests was carried out on individual 300 × 300 mm slabs of concrete, bituminous material and brick paving exposed to the atmosphere. The experiments were carried out on five samples:

The rainfall was measured by a tipping bucket rain gauge, and other meteorological parameters such as wind speed, temperature and relative humidity were also recorded. The samples were placed in plastic trays to collect the runoff, which was measured daily. In the case of the brick paving, the amount of infiltration through the joints was also recorded. A 50 × 50 mm cube of the same material was located next to each slab and was weighed daily to measure the change in moisture content in the material. The losses due to evaporation and other processes were estimated by using the water balance principle. The measurements were carried out over two periods in winter, totalling about 9 weeks.

It shows how the water balance for the horizontal concrete slab varied over a period of nearly 6 weeks. The relatively high infiltration rate at the start was due to the initial dryness of the slab. It shows how the moisture content in the concrete cube adjacent to the flat slab responded to rainfall. The concrete cube was originally dry and, like the slab, required several weeks to reach an equilibrium moisture content (equivalent to about 3 mm water depth). Thereafter, the moisture content did not vary greatly even after several days with no rainfall.

The water balances for five samples are compared. This shows that the slope of the concrete slab increased the runoff from 69 to 93%, with a corresponding reduction in evaporation. In the case of the brick paving, the most significant output was the infiltration through the sand-filled joints, with the runoff amounting to only 9% of the rainfall. The hot-rolled asphalt behaved in a manner similar to concrete, with about 2/3 of the rainfall appearing as runoff, while for the dense bituminous macadam 2/3 of the rainfall was detained on the surface and lost by evaporation, probably because of the surface texture. It should be noted that these experiments do not allow for the effect of micro-topography, which allows water to collect in larger scale depressions and hence be lost by evaporation, or for the effects of cracks and joints in the paving material, which would increase the amount of infiltration. In general, the results demonstrate that evaporation can be a significant factor in the urban water balance in the long term.

The combined effect of surface dryness and surface detention is that on several occasions there is not enough rainfall to produce any runoff. It shows the number of rain days where no runoff was recorded for the five different paved surface samples.

It should be noted that runoff was recorded on a daily basis: if the analysis had been carried out on the basis of individual rainfall events, it is likely that there would be more instances of events producing no runoff.