W-6

The discharge of a river shows much variation during a year and in the short term. It can fluctuate a lot in a matter of hours in response to periods of rain. Hydrological Cycle diagram
 * __How and why does the water in a river fluctuate?__**

A video on the hydrological cycle - []

The flood or storm hydrograph is used to show how a river responds to a period of rainfall. Rivers that respond swiftly to rainfall have a high peak and short lag time and are referred to as flashy. A lower peak and long lag time shows a delayed hydrograph. The amount of water in a river at any given point and time is known as the **discharge** which is measured in cumecs (cubic metres per second). This can be calculated by multiplying river velocity by channel volume at a given point and time.
 * __The storm hydrograph__**
 * __What is a hydrograph?__**

Hydrographs are graphs which show **river discharge** over a given period of time and show the response of a drainage basin and its river to a period of rainfall.

A storm hydrograph shows how a river's discharge responds following a period of heavy rainfall. On a hydrograph, the flood is shown as a peak above the base (normal) flow of the river. Analysis of hydrographs can help hydrologists to predict the likelihood of flooding in a drainage basin. The response of a river to a rainfall event can be measured in terms of the lag time - the time between peak rainfall and peak discharge. Rivers with a short lag time respond rapidly to rainfall events and are therefore more prone to flooding than rivers with a longer lag time River discharge does not respond immediately to rainfall inputs as only a little of the rainfall will fall directly into the channel. The river will start to respond initially through inputs from surface runoff (the fastest flow of water) and its discharge will later be supplemented through inputs from throughflow and groundwater flow The shape of a hydrograph is determined by the speed in which flood waters are able to reach the river. The nature of the drainage basin therefore has a great influence on the way a river responds to a river as it will determine the types and speeds of the flow of water to the river. The fastest route to the river is via **overland flow**. If most of the water in a drainage basin travels in this way, a river will respond quickly to heavy rainfall and the hydrograph shape will be 'peaky' (graph A) with steep rising and recessional limbs. The lag time will be short and there will be a greater risk of flooding. Where more water is able to pass into the soil and travel to the river via **throughflow / groundwater flow**, there will be a slower rise in discharge and the river will respond slower (graph B). The lag time will be longer and the risk of flooding will be much lower
 * __Variations in the shape of a hydrograph__**


 * __What are the factors which can affect river discharge?__**
 * Rock and soil type**
 * Permeable rocks ad soils (such as sandy soils) absorb water easily, o surface run-off is rare
 * Impermeable rock and soils (such as clay soils) are more closely packed. Rainwater can’t infiltrate, so water reaches the river more quickly
 * Pervious rocks (like limestone) allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles
 * Land use**
 * In urban areas, surfaces like roads are impermeable – water can’t soak into the ground. Instead, it runs into drains, gathers speed and joins rainwater from other drains – eventually spilling into the river
 * In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster 0 increasing discharge
 * Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
 * Rainfall**
 * The amount and type of rainfall will affect a river’s discharge
 * Antecedent rainfall s rain that has already happened. It can mean that the ground has become saturated. Further rain will then flow as surface run-off towards the river
 * Heavy continual rain, or melting snow, means more water flowing into the river
 * Relief**
 * Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate. On more gentle slopes infiltration is more likely.
 * Weather conditions**
 * Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into the river.
 * High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge
 * Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in


 * Key terms:**


 * Discharge:** the volume of water passing a given point in a river at any moment in time
 * Drainage basin:** area from which a river gets its water. The boundary is marked by an imaginary line of highland known as a watershed
 * Flood or storm hydrograph:** a line graph drawn to show the discharge in a river in the aftermath of a period of rain
 * Flashy:** a hydrograph that responds quickly to a period of rain so that it characteristically has a high peak and a short lag time
 * Relief**: height and slope of land
 * Impermeable:** rock that does not allow water to soak into it
 * Porous**: rock that has spaces (pores) between particles
 * Pervious:** rock that allows water to pass through it via vertical joints and horizontal bedding planes
 * Deforestation:** cutting down trees
 * Urbanisation:** the increase in the proportion of people living in cities, resulting in their growth