In catchment-based flood management, how do upstream source control measures, such as reforestation, directly affect downstream peak flood flows?

In catchment-based flood management, upstream source control measures like reforestation directly affect downstream peak flood flows by altering the hydrological cycle within the catchment, which is the area of land where all water drains into a common outlet. Reforestation involves planting trees and restoring forest cover in the upper parts of a river basin. These trees and their associated forest ecosystems modify how rainfall interacts with the land, thereby influencing the speed and volume of water reaching downstream river channels.

Firstly, tree canopies intercept rainfall, meaning a portion of the precipitation is caught on leaves and branches before it reaches the ground. This intercepted water either evaporates back into the atmosphere or slowly drips to the forest floor, delaying its entry into the soil and drainage system. This process, known as interception storage, reduces the immediate volume of water available for surface runoff during a rain event.

Secondly, forest soils have significantly higher infiltration rates compared to degraded or bare land. Tree roots create extensive networks of macropores and channels within the soil, increasing its porosity and ability to absorb water. The leaf litter and organic matter on the forest floor further enhance this by improving soil structure and acting as a sponge, allowing water to slowly percolate downward rather than flow across the surface. This increased infiltration means less water becomes surface runoff, which is water that flows over the land surface, contributing rapidly to river flows. Instead, more water infiltrates into the ground to replenish soil moisture and potentially groundwater stores, which are underground reserves of water.

Thirdly, trees actively absorb water from the soil through their roots and release it into the atmosphere as water vapor through their leaves in a process called transpiration. Together with evaporation from the soil and plant surfaces, this is known as evapotranspiration. Forests have higher evapotranspiration rates than many other land covers, meaning they return a greater proportion of the absorbed rainfall back to the atmosphere, further reducing the amount of water available for runoff.

The combined effects of increased interception, enhanced infiltration, reduced surface runoff, and higher evapotranspiration lead to a significant attenuation of flood peaks downstream. Attenuation refers to the reduction in the magnitude of a flood peak. By slowing down the movement of water from the land into the river system and reducing the total volume of water that rapidly enters the channels, reforestation extends the time to peak and reduces the peak discharge of a flood. The peak discharge is the maximum volume of water flowing past a point in a river per unit of time during a flood event. This results in a 'flattening' of the hydrograph, which is a graph showing the rate of flow (discharge) versus time past a specific point in a river. A flattened hydrograph signifies a lower and more prolonged flow, rather than a sharp, high-volume surge, thereby directly mitigating the severity of downstream flooding and reducing flood risk.