SAJC Cluster 2 Lecture 5 Notes (Drainage Basin Hydrology)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 5_2024/pg1 Lecture 5 Drainage Basin Hydrology KEY QUESTION: ✓ How does water move within a drainage basin system? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Input of water into the drainage basin system • Output of water from the drainage basin system • Storage of water in the drainage basin system • Movement of water through the drainage basin system Lecture Outline 5.1 Drainage Basin Hydrology 5.2 Input of Water into the Drainage Basin System 5.2.1 Precipitation 5.2.1 Snowmelt 5.3 Pathways and Storages in the Drainage Basin System 5.3.1 Interception Storage, Throughfall and Stemflow 5.3.2 Infiltration and Soil Moisture Storage 5.3.3 Percolation, Groundwater Storage and Baseflow 5.3.4 Throughflow 5.3.5 Overland Flow (a) Hortonian Overland Flow (HOF) (b) Saturation Overland Flow (SOF) 5.3.6 Channel Storage 5.4 Output from the Drainage Basin System 5.4.1 Evapotranspiration 5.4.2 River Discharge 5.5 Glossary

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 5_2024/pg2 5.1 Drainage Basin Hydrology • The drainage basin is the area of land surface from which water and sediment are transferred by individual channels that join up to form a network that eventually drains into an ocean, a sea or a lake (Fig. 1). • The boundary of a drainage basin is known as the watershed, and is simply the highest contour of the land surrounding a river. • Hydrology is the study of the distribution and movement of water both on and below the Earth's surface, as well as the impact of human activity on water availability and conditions. • Therefore, drainage basin hydrology is the study of how water moves or is transferred within a drainage basin. • Being a system defined by inputs, flows/pathways, stores and output working within the watershed, it is the basic unit for studying hydrological processes. The drainage basin system is an open system as water is not confined to a specific location and can move from one locality to the next at any given time. (See Fig. 2 and Fig. 3) o The water enters the system as precipitation or snowmelt and leaves as river discharge or as evapotranspiration. • The drainage water balance can be studied using the following simple equation, but it should be noted that the ‘balance’ is not a given as the basin is an open system: in which P is precipitation, E is evapotranspiration, R is surface runoff (or overland flow), and S reflects gains or losses due to changes in storage. (See subsequent sections) Fig. 1 Fig. 2 P = E + R ± S (possible th