CONCLUSION

Although this chapter does not pretend to replace a course in meteorology, it introduced the basic processes underlying the formation of precipitation and the methods used for analyzing it, which are based on two fundamental hydrological principles: return period and the intensity-duration-frequency curve. The chapter also touched upon the study of rain structure and how it is evaluated on a regional scale. These methods utilize several applied engineering techniques that will be discussed in detail in a separate book dedicated to hydrological engineering.

Geographical coordonnates x [m]

Geographical coordonnates y [m]

relative total rainfall [%] Simplon rainfall = 100%

Fig. 4.11 : Factor for the storm from September 21 till the 23 in 1993 occurring the north of the

Rhône watershed (Binn-Simplon). Areal reduction factor for the storm during the 21-23 of september in the north of Rhône catchment

EVAPORATION AND INTERCEPTION

n any attempt to analyze the water balance or understand the mechanisms of the water cycle, the processes of the interception, transpiration, and evapo- ration of water play a particular role. The practicing engineer requires a firm grasp of these processes in order to carry out any sort of drainage or irrigation project: without knowing the water losses resulting from interception and evapotranspiration, it is impossible to design an appropriate system. On the other hand, however, the scientific community is not always in agreement on the explanations for some of these phenomena. As a result, this topic is of crucial interest to anyone involved, to whatever degree, in the issues of water supply (for agriculture, for example), and also to more fundamental research efforts, given that there are so many aspects of the underlying mechanisms that need to be clarified. Although several complete books could be dedicated to this topic, this chapter addresses the basic elements, including the role these processes play, and the methods for quantifying and analyzing them from a hydrological rather than agricultural viewpoint. The chapter also contains many relevant references that will make it possible for readers to expand their knowledge.

5.1 INTRODUCTION

An analysis of the water balance on a continental scale shows that, with the exception of Antarctica, all continents evaporate some fraction of their precipitation: 55% for North America and Asia, and 75% for the African continent. This is an indication of just how significant this process is to the water budget, not just because of the volumes of water involved, but also due to its influence on the Earth’s climatic circulation.

Even on a smaller scale, we now know that evaporation from a lake or from the reservoir above a dam can play a significant role. For example, Lake Nasser, which was created by the Aswan High Dam, evaporates 11% of its volume of water each year,

which is equivalent to 14 km3 of water. This represents a loss of 3 meters of depth from

the surface each year. Meanwhile, losses due to evaporation from areas under vegetal cover, such as a forest, are far from being negligible. The Amazon rainforest, for example, loses up to 80% of incident precipitation through evaporation.

The water returns to the atmosphere in vapor form, not via a single mechanism, but through three distinct processes that will be addressed in this chapter. The first process involves the fraction of water intercepted by vegetation before reaching the ground, the second is the transpiration of plants, and the third is the evaporation of gravitational water.

5.1.1 Interception

An appreciable portion of the water from precipitation does not reach the soil. Instead, it is intercepted by various obstacles during the course of its trajectory. Although this trajectory is usually vertical, we also now know that there is a mechanism of horizontal interception of fog or dew; this phenomenon is more prominent in certain parts of the world (for example, forests located near the coast of Chile).

“

save communities that are deprived of adequate water supplies. This is the case for the town of El Tofo in Chile; when the local iron mine closed, the mining company took the town’s water distribution system with it when it left. However, the community has been able to survive by capturing fog, using nets on which the water condenses. With these nets, they collect a little more than four liters of water per day per square meter of net. This supplies the village with 25 liters per inhabitant per day.”

(Jacques Sironneau, Revue Française de Géoéconomie, 1998)

This chapter will focus on the vertical interception of precipitation, which is defined as the fraction of water that never reaches the ground. The definition employed here is the one used by hydrologists, and concerns only the intercepted water that evaporates, as opposed to precipitation that is temporarily intercepted before reaching the ground. This is why the term commonly used when discussing interception as it relates to the water budget is interception losses. Losses due to interception are expressed using the following equation:

(5.1)

where P_b, [mm] represents the total rainfall, i.e., the precipitation that reaches the

canopy or upper surface of the vegetal cover, P_c[mm] is the throughfall (rain that falls

through the plant canopy) and Pt[mm] is the stemflow (water that trickles down

branches and the trunks).

The processes of interception and evaporation are intimately connected. However, since interception relies on evaporation, this chapter will discuss the details of evaporation first, before returning to the role of interception in the water cycle

5.1.2 Evaporation and Transpiration

In the troposphere, which is the layer of the atmosphere closest to the Earth’s surface (it is approximately 2 to 3 kilometers thick), the ambient air is never dry; it contains a more or less significant amount of water in the gaseous state (water vapor) which comes from:

• Physical evaporation from the surfaces of open water (oceans, seas, lakes and waterways), from bare soils, and from surfaces covered by snow or ice.

• Transpiration from vegetation that releases water into the atmosphere. • Evapotranspiration from soil covered by vegetation.

The term “evapotranspiration” is the combined term for the transpiration and evaporation of water that occur in a vegetal environment. The two processses are combined in a single term because it is often difficult to differentiate them.

Evaporation and transpiration result from the transformation of water into its gaseous state, and therefore require energy. It is worth recalling that this transfor- mation results in cooling, and that the reverse process – condensation – releases heat- energy and is accompanied by a rise in temperature.

Evaporation, and more specifically evapotranspiration, plays an essential role in the study of the water cycle. As illustrated in Table 5.1, these mechanisms contribute to producing a significant percentage of incident precipitation, whether on the planetary or watershed scale.

Figure 5.1 summarizes in schematic form the various elements involved in the processes of interception, evaporation, and evapotranspiration, which are the main focus of this chapter. These elements include the incident precipitation at the surface of the plant canopy, transpiration of the vegetation, evaporation of intercepted water, throughfall of water that passes through the canopy, stemflow, and finally, evaporation of water in the soil and of the water taken up by the plants, part of which is lost through transpiration.

Table 5.1 : Relative magnitude of evapotranspiration (ET) in relation to incident

precipitation(P) at different spatial scales (P) at different spatial scales Magnitude of evapotranspiration

on the scale of planet: _{P = 116’000 km}3 _{ET = 72’000 km}3_/year 62 %

on the scale of a climatic zone:

P = 49’000 km3 ET = 27’800 km3/year 57 %

on a scale of Switzerland: _{P = 60 km}3 _{ET = 19.5 km}3_/year 33 %