Evaporation and Transpiration--
Some water is lost to the atmosphere by evaporation from water surfaces. This loss can hasten the disappearance of temporary floodplain lakes and dry out the soils of the floodplain. More water is lost by the transpiration of plants. Evaporation and transpiration are important in the general ecology of river basins as they create microclimates with local differences in rainfall. When floodplain forest is cleared, major changes in local climate and hydrology may occur.

Evaporation and transpiration also contributes to the speed at which floodplain waterbodies dry out, because up to two meters of water level may be lost to evaporation over the year; also, plants can seriously lower ground water reserves through transpiration.

The Hydrological Regime--
The water that flows in the river is more abundant at some times of the year than at others because of the seasonality of the rains. The history of the flow patterns in the river during any year is known as the hydrological regime. This is usually measured by taking water height at a series of set gauges down the river at set intervals, usually daily. The water levels at any one gauge can then be connected to form a continuous curve on a graph (called a hydrograph) to represent to hydrological regime of the river at that gauge. A series of gauging stations are distributed along the Mekong.

Hydrological regimes have two main components, the flood and low water, that are separated by the time at which the river overflows its banks at bankfull (defined as the level at which water is no longer contained in river channels, but spreads out over the floodplain). Although a typical hydrological curve involves a continuous change in water levels, four phases can be identified that are of great significance to aquatic plants and animals:

• Rising water;
• Peak flood;
• Falling water; and
• Dry season.

The period of rising, peak and falling water over bankfull represents the flood curve. The flood has different characteristics in different parts of the river depending on the nature of the rainfall and the size of the river basin. Small streams respond very rapidly to local rainfall and their flood curves are very spiky (also known as flashy). Flows in larger rivers result from the sum of the many small streams that feed them. As a result the hydrographs are much smoother, and in the largest rivers the curves are almost completely smoothed because they are the sum of the curves of many large rivers that often originate in different climatic zones.

The flood curve can be seen as a pulse of water that takes time to travel down the river because the waters spread out and are slowed by the floodplains. They are also stored in floodplain lakes and wetlands. Because of the delay, floods can arrive at places downstream some time after the main rainy season is over.

The characteristics, shape and timing of the flood curve is very important for the ecology and behaviour of most aquatic organisms, in particular the floodplain vegetation that depends entirely on the flood for its growth. Minor flood events in the main channel during low water are also important, especially in smaller rivers, as they can clean sediment from the gravels that are so important for many invertebrate and fish species. They also play an important role in stimulating the breeding and migratory behavior of some species.

The amount of rainfall varies from one year to another, and the intensity of the hydrological regimes varies accordingly. The frequency with which any hydrological regime (flood regime) recurs is usually expressed in terms of the number of years that it would take for a regime of that size to reappear. Thus you may hear of the ten-year flood or the hundred-year flood, which means that such a flood will only occur once every ten or a hundred years. Unfortunately, the real situation is not so regular, as extreme flood events tend to group and you may get more than one hundred year flood arriving in successive years.

Extreme flood events have been recorded several times in recent years in the Mekong basin, particularly in the year 2000. Extreme hydrographs are important as many of the changes that shape the river described in section 3 happen during the largest floods, when they are known as reset events, whereas the system may remain relatively stable for many years under more normal hydrographic regimes.

Although much attention is paid to the flood component of the hydrograph, the dry season portion is equally important, as the amount of water remaining in the river channels and floodplain lakes during the dry season determines whether any particular organism or group of organisms will survive. Drought years can be very damaging and have long lasting effects on animals and plants.

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