Annual hydrographs and river regimes

Overview

Annual hydrograph, simple and a complex river regime and regime of a river fed mainly by snowmelt.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an annual hydrograph, and what does it show?

   Model answer

   An annual hydrograph, also called a river regime, is a graph of the mean discharge of a river for each month across a whole year. The horizontal axis shows the twelve months and the vertical axis shows discharge in cumecs. It shows the seasonal pattern of flow - which months are the high-water and low-water seasons, how large the difference between them is, and how many peaks the year contains.

2. 2

   How does an annual hydrograph differ in purpose from a storm hydrograph?

   Model answer

   A storm hydrograph covers hours or days and shows the response of a river to one individual rainfall event, so it is used to judge how quickly a basin responds and how likely it is to flood after a particular storm. An annual hydrograph covers a whole year in monthly averages and shows the long-term seasonal rhythm of the river, so it is used to judge water availability, seasonal flood and drought risk, and the reliability of supply for abstraction and irrigation.

3. 3

   Distinguish between a simple and a complex river regime.

   Model answer

   A simple regime has one clear peak and one low-water season in the year, because a single dominant control such as summer snowmelt or a monsoon governs the flow. It is typical of smaller rivers within a single climatic zone. A complex regime has several peaks in the year, because a large river drains a basin crossing more than one climatic zone or relief type, so tributaries contribute their own peaks at different seasons - for example a spring snowmelt peak from mountain headwaters combined with a winter rainfall peak from lowland tributaries.

4. 4

   Why does a typical river in the north-west of the British Isles have its highest flow in winter?

   Model answer

   Rainfall is fairly evenly spread through the year, so the seasonal difference is driven by losses rather than by inputs. In winter, temperatures are low, the sun is weak and plants are dormant, so evapotranspiration is very small and almost all the rain becomes effective precipitation, filling soil and groundwater stores and passing on to the channel. In summer, higher temperatures and active vegetation mean high evapotranspiration takes much of the rain back to the atmosphere and soil moisture deficits absorb the rest, so discharge falls to its annual minimum in late summer.

5. 5

   Describe the regime of a river fed mainly by snowmelt.

   Model answer

   Discharge is very low through the winter, often close to zero, because precipitation is locked up as snow and ice in storage and the ground is frozen. It rises sharply in late spring or early summer as temperatures climb above freezing and the snowpack melts, producing a single tall peak. It then declines through late summer and autumn as the snowpack is exhausted, leaving the river dependent on any remaining glacier melt, rain and groundwater. The regime is simple, with a very large difference between maximum and minimum flow.

6. 6

   How does geology show up in a river regime?

   Model answer

   A river on permeable chalk or limestone has a subdued regime with a small difference between its highest and lowest monthly flows, because the large groundwater store releases water slowly and steadily all year, and the peak is delayed by several weeks or months after the wettest season while the aquifer fills. A river on impermeable rock has a much more variable regime that closely follows the pattern of rainfall, with high flows in the wet season and very low flows, or even a dry channel, in the dry season, because there is no groundwater store to smooth it out.

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