Discharge regime
The main factor affecting the regime is climate,[3] along with relief, bedrock, soil and vegetation, as well as human activity.
According to this, he termed three basic types:[6] Pardé split the simple regimes further into temperature-dependent (glacial, mountains snow melt, plains snow melt; latter two often called "nival") and rainfall-dependent or pluvial (equatorial, intertropical, temperate oceanic, mediterranean) categories.
[7] Beckinsale later more clearly defined the distinct simple regimes based on climate present in the catchment area and thus splitting the world into "hydrological regions".
And although discharge is still often used for showing seasonal variation, two other forms are more commonly used, the percentage of yearly flow and the Pardé coefficient.
However, temperature also plays a significant role, as well as the characteristics of its catchment area, such as altitude, vegetation, bedrock, soil and lake storage.
The primary factor affecting river regimes is the climate of its catchment area, both by the amount of rainfall and by the temperature fluctuations throughout the year.
If the water from precipitation is frozen, such as snow or hail, it has to melt first, leading to longer delays and shallower peaks.
[6] If the relief is rather flat, the snow will melt everywhere in a short period of time due to similar conditions, leading to a sharp peak about three months wide.
[6] However, if the terrain is hilly or mountainous, snow located in lowlands will melt first, with the temperature gradually decreasing with altitude (about 6 °C (43 °F) per 1000 m).
At exceptionally high altitudes, atmosphere is thinner so the solar insolation is much greater, which is why Beckinsale differentiates between mountain nival and glacial from similar regimes found at higher latitudes.
Vegetation growing in the river beds can drastically hinder the flow of water, especially in the summer, leading to smaller discharges.
[26] The most important aspect of the ground in this regard is the permeability and water-holding capacity of the rocks and soils in the discharge basin.
If the wet season is really pronounced, the rocks become saturated and fail to infiltrate excess water, so all rainfall is quickly released into the stream.
Examples of rocks with high water-holding capacity include limestone, sandstone and basalt,[26] while materials used in urban areas (such as asphalt and concrete) have very low permeability leading to flash floods.
On one side, water can be extracted either directly from a river or indirectly from groundwater for the purposes of drinking and irrigation, among others, lowering the discharge.
[14] On the other side, waste waters are released into streams, increasing the discharge; however, they are more or less constant all year round so they do not impact the regime as much.
Another important factor is the construction of dams, where a lot of water accumulates in a lake, making the minima and maxima less pronounced.
[7][14] The temperate pluvial regime (Beckinsale symbol CFa/b[27]) usually has a milder minimum and the discharge is quite high also during the summer.
Meanwhile, the Mediterranean regime (Beckinsale symbol CS[27]) has a more pronounced minimum due to a lack of rainfall in the region, and rivers have a noticeably smaller discharge during summer, or even dry up completely.
Beckinsale distinguished another pluvial regime, with a peak in April or May, which he denoted CFaT as it occurs almost solely around Texas, Louisiana and Arkansas.
[3] It occurs in Africa around Cameroon and Gabon, and in Asia in Indonesia and Malaysia, where one peak is in October/November/December and another in April/May/June, sort of being symmetrical for both hemispheres.
Nival regime is characterized by a maximum which is contributed by the snow-melt as the temperatures increase above the melting point.
They occur in regions with continental and polar climate, which is on the Southern Hemisphere mostly limited to the Andes, Antarctica and minor outlying islands.
Beckinsale differentiates six plain nival to nivo-pluvial regimes, mainly based on when the peak occurs.
If the nivo-pluvial peak occurs later, in April or May (October or November on the Southern Hemisphere), followed by the discharge of the other month, the regime is transitional nival[28] or DFb/c.
In parts of Russia and Canada and on elevated plains, the peak can be even later, in May or June (November or December on the Southern Hemisphere).
[24] Beckinsale also added another category, Dwd, for rivers that completely diminish during the winter due to cold conditions with a sharp maximum in the summer.
Pardé and Beckinsale both assigned only one category to the mountain nival regime (symbol HN[24]), but Mader distinguishes several of them.
[28] The nivo-glacial regime occurs in areas where seasonal snow meets the permanent ice sheets of glaciers on top of mountains or at higher latitudes.
This is usually at altitudes over 2,500 m (8,202 ft 1 in),[14] but it can also happen in polar climates which was not explicitly mentioned by Pardé, who grouped both categories together.
