Discharge regime


Discharge regime, flow regime, or hydrological regime is the long-term pattern of annual changes to a stream's discharge at a particular point. Hence, it shows how the discharge of a stream at that point is expected to change over the year. It is thus the hydrological equivalent of climate. The main factor affecting the regime is climate, along with relief, bedrock, soil and vegetation, as well as human activity.
Like general trends can be grouped together into certain named groups, either by what causes them and the part of the year they happen or by the climate in which they most commonly appear. There are many different classifications; however, most of them are localized to a specific area and cannot be used to classify all the rivers of the world.
When interpreting such records of discharge, it is important to factor in the timescale over which the average monthly values were calculated. It is particularly difficult to establish a typical annual discharge regime for rivers with high interannual variability in monthly discharge and/or significant changes in the catchment's characteristics.

Overview

Maurice Pardé was the first to classify river regimes more thoroughly. His classification was based on what the primary reasons for such pattern are, and how many of them there are. According to this, he termed three basic types:
  • Simple regimes, where there is only one dominant factor.
  • Mixed or double regimes, where there are two dominant factors.
  • Complex regimes, where there are multiple dominant factors.
Pardé split the simple regimes further into temperature-dependent and rainfall-dependent or pluvial categories.
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". His main inspiration was the Köppen climate classification, and he also devised strings of letters to define them. However, the system was criticised as it based the regimes on climate instead of purely on discharge pattern and also lacked some patterns.
Another attempt to provide classification of world regimes was made in 1988 by Heines et al., which was based purely on the discharge pattern and classified all patterns into one of 15 categories; however, the determination is sometimes contradictory and quite complex, and the distinction does not differentiate between simple, mixed or complex regimes as it determines the regime solely on the main peak, which is contradictory to commonly used system in the Alpine region. Hence, rivers with nivo-pluvial regimes are commonly split into two different categories, while most pluvio-nival regimes are all grouped into a single category along with complex regimes – the uniform regime, despite showing quite pronounced and regular yearly pattern. Moreover, it does not differentiate between temperature-dependant and rainfall-dependant regimes. Nonetheless, it added one new regime that was not present in Beckinsale's classification, the moderate mid-autumn regime with a peak in November or May. This system too, is very rarely used.
In later years, most of the research was only done in the region around the Alps, so that area is much more thoroughly researched than others, and most names for subclasses of regimes are for those found there. These were mostly further differentiated from Pardé's distinction. The most common names given, although they might be defined differently in different publications, are:
  • Glacial, for regimes where most of water is due to melting of snow and ice and the peak occurs in late summer.
  • Nival, with a peak in late spring or early summer and still high importance of snow-melt.
  • Pluvial, which is purely based on seasonal rainfall and not on snow. A peak is usually in winter, although it can occur at any point along the year. If it occurs in the time of monsoons, it is sometimes called tropical pluvial.
  • Nivo-pluvial, with a nival peak in late spring and a pluvial peak in the fall. The main minimum is in winter.
  • Pluvio-nival, which is similar to nivo-pluvial, but the nival peak is earlier and the main minimum is in summer, not in winter.
  • Nivo-glacial, for regimes sharing characteristics of glacial and nival regimes and a peak in mid summer.
The Pardé's differentiation of single regimes from mixed regimes is sometimes rather considered to be based on the number of peaks rather than the number of factors as it is more objective. Most of nival and even glacial regimes have some influence of rainfall and regimes considered pluvial have some influence of snowfall in regions with continental climate; see the coefficient of nivosity. The distinction between both classifications can be seen with the nivo-glacial regime, which is sometimes considered as a mixed regime, but is often considered as a simple regime in more detailed studies. However, many groupings of multiple pluvial or nival peaks are still considered a simple regime in some sources.

Measurement of river regimes

River regimes, similarly to the climate, are compounded by averaging the discharge data for several years; ideally that should be 30 years or more, as with the climate. However, the data is much scarcer, and sometimes data for as low as eight years are used. If the flow is regular and shows very similar year-to-year pattern, that could be enough, but for rivers with irregular patterns or for those that are most of the time dry, that period has to be much longer for accurate results. This is especially the problem with wadis as they often have both traits. The discharge pattern is specific not only to a river, but also a point along a river as it can change with new tributaries and an increase in the catchment area.
This data is then averaged for each month separately. Sometimes, the average maximum and minimum for each month is also added. But unlike climate, rivers can drastically range in discharge, from small creeks with mean discharges less than 0.1 cubic meters per second to the Amazon River, which has average monthly discharge of more than 200,000 cubic meters per second at its peak in May. For regimes, the exact discharge of a river in one month is not as important as is the relation to other monthly discharges measured at the same point along a particular river. 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.
Percentage of yearly flow represents how much of the total yearly discharge the month contributes and is calculated by the following formula:
where is the mean discharge of a particular month and is the mean yearly discharge. Discharge of an average month is and the total of all months should add to 100%.
Even more common is the Pardé coefficient, discharge coefficient or simply the coefficient, which is more intuitive as an average month would have a value of 1. Anything above that means there is bigger discharge than average and anything lower means that there is lower discharge than the average. It is calculated by the following equation:
where is the mean discharge of a particular month and is the mean yearly discharge. Pardé coefficients for all months should add to 12 and are without a unit.
The data is often presented is a special diagram, called a hydrograph, or, more specifically, an annual hydrograph as it shows monthly discharge variation in a year, but no rainfall pattern. The units used in a hydrograph can be either discharge, monthly percentage or Pardé coefficients. The shape of the graph is the same in any case, only the scale needs to be adjusted. From the hydrograph, maxima and minima are easy to spot and the regime can be determined more easily. Hence, they are a vital part for river regimes, just as climographs are for climate.

Yearly coefficient

Similarly to Pardé's coefficient, there are also other coefficients that can be used to analyze the regime of a river. One possibility is to look how many times the discharge during the peak is larger than the discharge during the minimum, rather than the mean as with Pardé's coefficient. It is sometimes called the yearly coefficient and is defined as:
where is the mean discharge of the month with the highest discharge and is the mean discharge of the month with the lowest discharge. If is 0, then the coefficient is undefined.

Annual variability

Annual variability shows how much the peaks on average deviate from the perfectly uniform regime. It is calculated as the standard deviation of the mean discharge of months from the mean yearly discharge. That value is then divided by the mean yearly discharge and multiplied by 100%, i.e.:
The most uniform regimes have a value below 10%, while it can reach more than 150% for rivers with the most drastic peaks.

Grimm coefficients

Grimm coefficients, used in Austria, are not defined for a single month, but for 'doppelmonats', i.e., for two consecutive months. The mean flow of both months – January and February, February and March, March and April, and so on – is added, still conserving 12 different values throughout the year. This is done since for nival regimes, this better correlates to different types of peak. They are defined as follows:


where.

Coefficient of nivosity

Pardé and Beckinsale determined whether the peak is pluvio-nival, nivo-pluvial, nival or glacial based on the fact what percentage of the discharge during the warm season is contributed by the melt-water, and not by the time of the peak as it is common today. However, it has been calculated for few rivers. The values are the following:
  • 0–6%: pluvial
  • 6–14%: pluvio-nival
  • 15–25%: nivo-pluvial
  • 26–38%: transition to nival
  • 39–50%: pure nival to nivo-glacial
  • more than 50%: glacial

    Factors affecting river regimes

There are multiple factors that determine when a river will have a greater discharge and when a smaller one. The most obvious factor is precipitation, since most rivers get their water supply in that way. 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. An important factor is also the human factor as humans may either fully control the water supply by building dams and barriers, or partially by diverting water for irrigation, industrial and personal use. The factor that differentiates classification of river regimes from climate the most is that rivers can change their regime along its path due to a change of conditions and new tributaries.