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 that RHS > LHS and RHS < LHS)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 5_2024/pg3 Fig. 3 A diagrammatic representation of drainage basin hydrological processes 5.2 Input of Water into the Drainage Basin System 5.2.1 Precipitation • Precipitation refers to the conversion and transfer of moisture in the atmosphere to the land. It forms when vapour in the atmosphere cools to its dew point and condenses into tiny water droplets or ice particles to form clouds. Eventually these droplets or ice particles aggregate, reach a critical size and leave the cloud as precipitation. • In the Tropics, precipitation is mainly in the form of rain (and snow at high altitudes) which provides the initial input of water into the hydrological system. It is also an important factor affecting how rivers behave. • Depending upon the size of the drainage basin, precipitation totals will influence the input (the potential amount of water which can enter a system) and the output (the eventual channel flow). • The basic understanding is that precipitation varies over space and time at all scales. This is largely climate-dependent (type, volume, intensity and timing; see Lect 6 for more detailed explanation). It is important also that we separate water delivery to a river into two stages: (1) the precipitation as it falls (i.e. direct input), and (2) what happens to it when it arrives at the surface (i.e. entry into river channel via the various pathways). As only around 10% of global precipitation arrives as channel precipitation, i.e. falls directly on to the river surface, the journey of this second stage is crucial. 5.2.2 Snowmelt • At high altitude places in the Tropics, the precipitation in a drainage basin may fall as snow rather than rain. o For instance, at the summit of Mt Kilimanjaro and the Himalayas, there is a considerable buildup of snow cover during winter which can be considered as surface storage (see Section 5.3.2). Here, snow acts as a temporary store of water in the drainage basin. This delays delivery of the water input to the system, lowering river levels (see Section 5.3.7).

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 5_2024/pg4 • Snow cover can be significant in influencing run-off when it melts, such as when in spring or summer. Once a thaw sets in, the volume of melt water can be very high and the speed at which water will rush into the channel is fast, making it prone to flooding (see Lect 7). 5.3 Pathways and Storages in the Drainage Basin System • Pathways (PW), consisting of different types of flows, refer to the paths taken by water as it travels from one storage to another. • Storages (S) are a vital control on the operation of the system. Storage refers to the parts of the system that hold or retain water for periods of time. o Each one can be envisaged as a sponge, capable of absorbing and holding water as it travels through the system, but liable to begin to release it while it is filling with water, and eventually becoming so saturated that it releases as much as it takes in. • Pathways and storages in a water basin are summarised in Fig. 2 and Fig. 3. 5.3.1 Interception Storage (S), Throughfall (P) and Stemflow (P) • The amount of incoming precipitation input which reaches the ground surface directly depends not only upon its type, volume, intensity and timing, but also upon surface cover. • Interception is the part of the rainfall that is intercepted by the earth’s surface (Fig. 4); Earth’s surface here includes everything that becomes wet after a rainfall event. It includes: vegetation, soil surface, litter, build-up surface, etc. o Interception can amount up to 15-50% of precipitation. o Interception loss: Water that is retained and later lost as evaporation, after the storm. o Interception storage: refers to the water that is caught and stored/retained on the surfaces. Fig. 4

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 5_2024/pg5 • Also, some raindrops will run down branches and the trunks of trees as stemflow. 5.3.2 Infiltration (P) and Soil Moisture Storage (S) • When rain falls on to the land surface, a proportion will, under most circumstances, sink directly into the soil; this is known as infiltration. So, infiltration is the vertical downward flow of water from surface storage to soil moisture storage. (See Fig. 3) o The ability of a soil to allow the entry of water is referred to as its infiltration capacity; this is expressed in terms of the depth of wa