# Gw discharge to a river is commonly referred to as

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table is towards the river (Fetter, 1994). GW discharge to a river is commonly referred to as ‘base flow’ (Fetter, 1994). Baseflow can be defined as the withdrawal movement of water from GW storage to feed rivers. Similarly, it is defined as the amount of river flow that is derived from GW storage (Hiscock, 2005). Figure 2: Gaining river reach (Winters et al, 1998) 2.1.2 Losing River A river which loses water to the underlying GW is called an ‘influent’ river and there are two types. The first type is illustrated below. If the water table is at a lower elevation than the river, focused recharge
4 from the river to the underlying GW will occur. This is because the hydraulic gradient of the water table to the river has been reversed (Healy, 2005). Figure 3: Losing river reach - hydraulically connected to GW (Winters et al, 1998) The second type of losing river is illustrated in Figure 4. If the water table is lower than the river and has an unsaturated zone in between, it is hydraulically disconnected from the GW. The water lost from the river is only considered ‘potential’ recharge to the GW. This is because all of the water may not reach the water table due to some being evaporated or travelling as interflow through the soils (Hiscock, 2005). Figure 4: Losing river reach - disconnected from GW (Winters et al, 1998) 2.1.3 Gaining and losing river During (and after) a large precipitation event when river flows rise, many rivers or just sections of the river can quickly change from gaining water to losing water. Once time has passed and the river flow has returned to normal, the river can then return to gaining water (Winters et al, 1998).
5 3 Theory 3.1 River Runoff Hydrograph A river hydrograph is a graph the shows river flow (discharge) over time. It is produced from measurements of river discharge, which are obtained from river gauging stations. The annual hydrograph of the Feshie (an upland Scottish river) is shown in figure 5 as an example. The data is daily gauged flows. A Unit Hydrograph (UH) is a graphical representation of direct runoff resulting from one unit of effective rainfall of a specified duration, generated uniformly over the basin area at a uniform rate. This procedure for determining unit hydrographs from observed rainfall and runoff data, and the application of such data for the derivation of runoff from a basin when only rainfall data was available was first explained by L-K Sherman in 1932 (Sherman, 1932) and (Zlatunova, 2002),. Jones (2006) in Salami, et. al., 2009 reported that the unit hydrographs method is suitable for wetlands that are not beyond 5000 km2 in size. Runoff is influenced by the area, vegetation cover, ground slope and terrain, orientation, shape, altitude, soil type and also the pattern of streams within the basin. Figure 5: Feshie annual hydrograph - gauged daily flow (NRFA, 2016) 3.1.1 Runoff Hydrograph Components Hydrograph components include rising limb, recession limb, peak, direct runoff, and base flow. The volume represented by the area of a hydrograph of surface runoff must equal the volume of the rainfall less infiltration, baseflow and other losses (Sherman, 1932). The shape of the hydrograph reflects all

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