25.1. Definition and Concept

Depression storage is defined as the amount of water that collects in surface depressions during rainfall and does not contribute to surface runoff. It must be filled before any runoff can occur. It is considered part of initial losses in hydrological accounting.

Depression storage refers to the water that accumulates in small surface depressions like puddles, holes, or uneven ground after it rains. Before any water can move into rivers or lakes as runoff, these depressions need to fill up. This concept is crucial in hydrology because it impacts how much water is available for soaking into the ground (infiltration) and how much will be lost before runoff begins. Losses in hydrology include surface water that is intercepted by vegetation, water that seeps into the ground, and the water held in depression storage.

Imagine a sponge that soaks up water. Before the sponge can overflow, it must fill up completely. Similarly, before rainwater can flow away as runoff into streams or rivers, it first fills the little 'sponge-like' depressions on the ground.

• Initial abstraction = depression storage + interception + infiltration (before runoff starts).

Initial abstraction is a key concept in hydrological studies. It encompasses all the water that is lost in the early stages of a rain event before any excess water begins to flow away as runoff. This includes water stored in depressions, water that gets caught by plants and trees (interception), and water that soaks into the soil (infiltration). Understanding these factors helps in managing water more efficiently and predicting the behavior of water in the environment.

Think about how a sponge can hold onto water before it starts dripping. The amount of water it can hold corresponds to depression storage. Any water that is lost to the sponge (infiltration) or is absorbed by a surface (interception) before it starts dripping out again represents initial abstraction.

• Typical examples: water in potholes, ploughed fields, ruts, low-lying areas, etc.

Depression storage can be found in various forms in our environment. For instance, after it rains, potholes on roads can collect water, just like how fields that have been ploughed can hold small amounts of water in their grooves. Low-lying areas are also natural depressions that gather rainwater. Recognizing these examples helps in understanding how water is stored temporarily before it drains away or evaporates.

Consider a child playing in a mud puddle. The puddle represents a depression where rainwater collects. As the child plays, the puddle gets filled with more water until it can no longer hold it, leading to the overflowing of water – this visualizes how natural depressions function in managing water.

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