SAJC Cluster 2 Lecture 6 Notes (Variations in Balance, Stores and Pathways in Drainage Basin System)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 6_2024/pg1 Lecture 6 Variations in Balance, Stores and Pathways in a Drainage Basin System KEY QUESTIONS**: ✓ How does the balance between input and output in the drainage basin system vary over time and space? ✓ How and why do stores in the drainage basin system vary in the tropics? ✓ How and why do pathways in the drainage basin system vary in the tropics? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Variations in the balance between input and output over time and space in the tropics due to natural factors • Variations in stores in the drainage basin systems in the tropics over time and space due to natural and human factors • Variations in pathways in the drainage basin systems in the tropics over time and space due to natural and human factors **Note that these three key questions can only be answered using materials in both Lect 5 and Lect 6. Lecture Outline 6.1 Some basics: River Discharge and the Hydrograph Box 1: Different ways of interpreting river discharge 6.1.1 Components of the Hydrograph 6.1.2 Flashy vs Attenuated Hydrograph Factors affecting Drainage Basin Hydrology 6.2 Climate 6.2.1 Precipitation (a) Type (b) Seasonality and Amount (c) Intensity of Rainfall (d) Duration of Rainfall 6.2.2 Temperature Box 2: The effect of climate on the balance between input and output in a drainage basin 6.3 Geology 6.4 Soil Condition 6.5 Slopes of the Basin 6.6 Changes in Vegetation Cover

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 6_2024/pg2 6.1 Some basics: River Discharge and the Hydrograph • An important aspect of hydrology is how a drainage basin (recall Lect 5) reacts to a period of rain. This is important because it can be used to predict flood risks (see Lect 7) and make the necessary precautions to avoid damage to property and loss of lives. • The response of a river can be studied by using the hydrograph, which is a means of showing the discharge of a river at a specific gauging station over time (see the small rectangle within the basin in Fig. 1). • Hydrographs show variations in the discharge of a river for a particular rainfall event and for a short period of time, usually hours or days (rather than weeks or months). • A hydrograph (see Fig. 3 next page) only describes what happened to the river discharge (see Box 1, and also Lect 5 Section 5.4.2). We need to be able to interpret the graph in order to explain what happened (to the other stores and pathways). In turn, this will help us to predict and forecast what might happen. Box 1: Different ways of interpreting river discharge − Discharge is the volume of water passing through a particular point of the main river or channel in a unit of time. Mathematically, it is expressed as Q = AV, where Q is the discharge (m3/s), A is the cross-section area (m2) of the channel, and V is the velocity (m/s) of the channel flow. − This volume of water originates as precipitation which reaches the river or channel by surface runoff (overland flow), throughflow and baseflow (see Fig. 2A and 2B). Discharge is therefore also understood this way: Discharge = Overland flow (or Surface flow) + Throughflow + Baseflow − Recall from Lect 5 that, if P (precipitation) is the input, E (evapotranspiration) and R (channel flow/discharge) are the outputs, and S refers to the various storages within the basin, then P = E + R ± S. This reminds us that discharge (whether represented either by Q or R) forms an important part of the basin water balance. Fig. 1 Gauging Station Fig. 2A Fig. 2B

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 6_2024/pg3 6.1.1 Components of the Hydrograph • In order to show the relationship between the precipitation (input) and the channel discharge (output) past the gauging station, most (not all) hydrographs will include the rainfall graph (see Fig. 3). This relationship is important because it determines the speed and scale of the rise in discharge, and therefore the likelihood of flooding (see Lect 7). • The discharge has three constituent parts (see Box 1 again): o When the rainstorm begins, the river discharge does not respond immediately to rainfall inputs as only a little of the rainfall will fall directly into the channel while the rest falls elsewhere in the basin which either gets intercepted, stored on the surface, or infiltrated, and takes time to reach the channel. o Surface flows reach the channel faster than the sub-surface flows. The river will start to respond initially through inputs from overland flow (the fastest flow of water) and its discharge will later be supplemented through inputs from throughflow and baseflow. • When the initial overland flow and, later, throughflow eventually reach the channel, there is increase in discharge in the channel. This rise in discharge (volume of water or water level) in the channel after a rainfall event starts is indicated by the rising limb in the hydrograph. o The steeper the rising limb means the faster the channel’s response to rainfall i.e. water reaches the channel very quickly. This is usually due to overland flow and throughflow. (Collectively, overland flow and throughflow are known as storm flow.) o Conversely, the gentler the rising limb implies a slower response due to little storm flow. • Baseflow is very slow to respond to a storm, but by continually releasing groundwater it maintains the river’s flow during periods of low precipitation. Indeed, baseflow is more significant over a longer period of time than an individual storm. • The peak discharge (or peak flow) is the maximum discharge in the channel which occurs when the river reaches its highest level for a particular rainfall event. Fig. 3 Components of the hydrograph 10 20 30 40 50 0 10 20 30 40 50 Rainfall (mm) Discharge (m3/s) time 1200 (day 1) 0000 (day 2) 1200 (day 2)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 6_2024/pg4 • Recall that storages delay the conversion of input to output, of precipitation to runoff. The time difference between maximum precipitation and peak discharge in a drainage basin is referred to as the lag time. o The lag time reflects the time needed both for the rain to generate overland flow, and for that overland flow to pass downslope into the channel. • The falling limb is the segment of the hydrograph where water is still reaching the channel but discharge is decreasing and river level is falling. This segment is usually less steep than the rising limb as although the rainfall has stopped, the system of storages might be sufficiently well-filled to be discharging continuously (via throughflow and baseflow) and so response to the decline in input is much more gradual. • By the time all the water from the rainstorm has passed through the channel at a given location, the river will return to its baseflow level (i.e. the level before the storm) unless there has been another storm within the basin. • Finally, on the hydrograph, bankfull discharge (BD) occurs when a channel is completely filled with water from the top of one bank to the other. In other words, when peak discharge (PD) or water level reaches the top of its channel, any