# Drain

Outflow , flow and inflow (scientific / mathematical abbreviation Q ) is the volume of water in hydrology that leaves or enters a given catchment area under the action of gravity within a certain time. In hydrogeology , runoff according to DIN 4049-1 is a water that moves under the influence of gravity on and below the land surface. The use of the term differs in practice in the hydrogeological , hydrological and hydrographic specialist literature partly due to different reference and observation parameters. The DIN 4049-3 Hydrology, Part 3 defines the term runoff and associated components universal.

## Basics

Because measurements in hydrography are based on a level (i.e. a measuring point), measurements are always made downstream and this level is used for the upstream drainage area (the so-called upper water). The volume of water that passes the measuring point is the flow at the level or discharge of the river basin . The inflow - into a lake, for example - is hydrographically considered “the outflow into the lake at the confluence ”; analogously, the flow through an area is the volume of water that “flows off” into the underwater per unit of time, passing the level, and the pouring of a spring its runoff into the surface water .

### Drainage components

According to DIN 4049-3, the total runoff consists of the surface runoff, the intermediate runoff and the base or groundwater runoff. As surface runoff (Qo) is the portion of the effluent amount of water, the above ground without a soil passage receiving water flows in. Some authors also refer to this runoff proportion as "rapid direct runoff", since the water generally reaches the receiving water after a few hours after a precipitation event.

The runoff is influenced by numerous physiogeographical and hydroclimatic regime factors such as a. Morphology , soil , land use and precipitation , evapotranspiration and the formation of the river or stream bed, which interact with each other.

When it penetrates the ground, the rainwater either gets directly into the groundwater-filled area or into a low-groundwater layer that is located above the actual aquifer . The infiltrated water is dammed in the low groundwater layer and mostly flows, following the gradient, with a time delay, within a few days, to the receiving water. This outflow component as interflow or interflow (Q) , respectively. Depending on the flow times, a distinction is made between an immediate (fast) and a delayed (slow) intermediate drain.

Surface and intermediate runoff together form the direct runoff (QD) in accordance with DIN 4049-3 , which thus includes all runoff components that reach the receiving water with only a slight delay after precipitation. As base runoff (QB) or groundwater-borne runoff (QGW), DIN 4049-3 defines the part of the runoff that cannot be counted as direct runoff. The flow velocities of the base runoff are generally significantly lower than those of the direct runoff components. The long-term base runoff largely corresponds to the formation of new groundwater . The groundwater moves towards the next receiving water according to the hydraulic potential . The so-called dry weather runoff from the receiving waters is a measure of the new formation of groundwater after long periods without precipitation .

### Mathematical basics

The discharge  Q indicates the volume  V that passes a complete cross-sectional area F in a flowing water in a certain time  t , i.e. H. the volume flow of the water body by  F . Its unit is therefore m 3 / s:

${\ displaystyle Q = {\ frac {V} {t}} = \ int _ {F} v \ operatorname {d} F}$
v ... flow velocity
F ... flowed through area.

The volume, colloquially the "amount of water", is not measured directly, but accessed indirectly:

${\ displaystyle V = Q \ cdot t = {\ bar {v}} t \ cdot F _ {\ mathrm {P}}}$

As a first approximation, the time-dependent values ​​are to be assumed as mean values :

v̅̅ … mean flow velocity at the gauge
v̅̅ t = s … mean flow path
F P … area of ​​the river bed profile , reference area of ​​the gauge

Since the profile at the gauge is known, the flow area results from the gauge alone: ​​The dependence of the river width b on the water level h or level P can be read from the cross section of the water body

${\ displaystyle F _ {\ mathrm {P}} = F (P)}$

This means that Q and V can be determined from the mean flow velocity and the water level - the former is also an empirical value about the water level and the water temperature (which determines the viscosity of the water), the conditions in the upper and lower water and the material properties of the bed (roughness , resistance to flow), thus the fluid mechanical characteristic depends - Idealisierungsannahmen are about Newton linear decrease in flow rate over the normal of the direction of flow ( elementary set of fluid friction , toughness coefficient ) in laminar flows , after the average flow velocity half of the (readily measurable) velocity at the surface is: It is therefore advisable to set up water level measuring points in the most uniform sections of the river.

## Measurement

In contrast to the water level, the flow is difficult to measure and is therefore usually calculated from the water level and the flow velocity. However, there are also methods to measure the flow directly.

Is the change in the outflow cross-section with rising water level h and water level P is known, it can be from a discharge curve (h / Q-diagram) and level ratio (P / Q) plot set up, which represents the relationship between discharge and water level and thus enables indirect discharge measurement via the water level. For a V-shaped profile, the relationship is exactly dependent on the fourth power of the water level. With flatter profiles, the flow rate increases a little faster, with U-shaped profiles a little more slowly. In addition, at the same water level, the flow rate depends on the water temperature: at 25 ° C, twice as much water flows as at 0 ° C.

## Discharge parameters

The following values ​​are important parameters for hydraulic engineering . When expanding a river, when dimensioning retention volumes and tipping structures, it must be proven that there is no flood over time periods, the length of which is greater, the more sensitive the adjacent use is: long periods of time for built-up banks, shorter periods for agricultural use.
Also flood warning levels are usually determined on the basis of the discharge parameters.

• Lowest known discharge (NNQ): This parameter indicates the lowest discharge that was recorded at the measuring point. For this value, the time of occurrence must be specified.
• Lowest outflow of similar periods of time (NQ) in the period under consideration: In contrast to the NNQ, this value reflects the lowest outflow of similar periods of time (month, half-year, year) within a period of observation. The time period and the period of time are added to the specification. If no period was given, the full year is meant (also: MJQ )
For example, NQ 1971/1980 is the lowest discharge from the years 1971 to 1980, WiNQ 1971/1980 the lowest in the winters 1971 to 1980, DezNQ 1971/1980 the lowest discharge occurred in the December
months of the years 1971 to 1980.
• Mean low water discharge (MNQ) in the period under consideration: This parameter is the arithmetic mean of the lowest discharge (NQ) for similar periods of time for the years of the observation period. As with NQ, the period of time and the observation period must be added to the information, such as mean annual low water discharge (MJNQ)
For example, MNQ 1971/1980 is the mean of the NQ values ​​from the 10 individual years 1971 to 1980.
• Mean runoff (MQ): The mean runoff MQ is the average runoff, measured over a normal year - i.e. the long-term average, in hydrography based on the runoff year , which generally begins in the temperate climate zones in autumn, around an entire winter cycle capture. Late snowmelts and especially glaciers delay the runoff of winter precipitation until summer.
The annual runoff (the “annual runoff” in m³ / year) results in the total annual “amount” (the volume of water) that the drainage area feeds into the one below. The mean discharge MQ of a drainage system is then calculated as the time average of the regular water volume over the year.
• Mean flood discharge (MHQ): This parameter is the arithmetic mean of the highest discharges (HQ) for similar periods of time for the years of the observation period. As with NQ, the time period and the observation period must be added to the information.
• Highest flood discharge ever measured (HHQ): Historically documented maximum flood
• Mathematically highest flood discharge (RHHQ): The hydraulic engineering reference value of the highest flood
• Flood discharge with annuality n (HQn, or T, HQT): discharge with a certain return probability (in years: annuality)
Common sizes are HQ1, HQ2, HQ5, HQ10, HQ25, HQ30, HQ50, HQ100, HQ300, HQ1000 for statistically 1-year (expected every year), 2, 5, 10, 25, 30-year (usual meteorological interval), 50-, 100-year (" century flood "), 300-year ("since time immemorial") and 1000-year flood (the latter correspond to "since the beginning of the records", which depending on the area in the earlier modern times or go back to the Middle Ages). HQ5000, for example, is less common, such events can only be detected geologically.

### calculation

The calculation of these quantities is a very complex matter, which is one of the central tasks of modern hydrography :

• A certain time interval must be used as a basis for the mean values; 30 or 40-year measurement series are common. However, these turn out to be too short for exceptional events; longer, closed data series are often not available. In addition, the measurement intervals differ from region to region, which makes the comparability and overall calculation of a water system difficult.
• The discharge estimate becomes very imprecise due to excessive flooding. In addition, because of their rarity, only very few exact measurements are usually available for such extreme events.
• Annual low water levels and high water levels are recorded in the average values, but not every event of the century. In the time of climate change in particular, the measurement intervals used earlier, which end around 2000, are less meaningful than they are today; they probably only reproduce the warming phase after the small climate minimum of the mid-forties.
• In addition, the normal water discharge can only be determined if the annuality of an exceptional event can be assumed. Conversely, however, the annuality is determined on the basis of the normal value, that is, if extreme events pile up, the mean value shifts. From a scientific point of view, therefore, the calculation of the values ​​requires permanent adjustment, while hydraulic engineering flood protection requires long-term reliable values. In the past, protective structures were dimensioned for a hundred-year flood, today, therefore, significantly higher values ​​are used, in the range of a 500 to 1000 year event.

As a result, these values ​​are usually no longer measured directly, but are calculated using precipitation-runoff models (NA models) from the parameters of the catchment area and the course of annual precipitation, which is recorded at a conveniently located measuring station. Larger values ​​are calibrated at historical high water marks.

## Drainage channel and drainage regime

The discharge profile of the level results from the flow (Q / t diagram). It shows the behavior (the reaction) of the flowing water on the basis of the measured flow, as it results from the weather situation and the discharge conditions of the catchment area . You can look at the hydrographs of a discharge event or the annual course of the discharge - all flow values ​​for the discharge year can be entered in a diagram. The hydrograph forms the basis for characterizing the runoff of a body of water at this point on the basis of typical flows (low water, high water courses).

According to the long-term mean runoff hydrograph, the water body is assigned to a certain runoff regime , a classification class of recurring patterns (seasonal characteristics, such as snowmelt, summer glacial runoff ; characteristics of the high mountains or the plains, etc.) depending on the climate .

Models for flood forecasting or water management planning in a catchment area are based on the corridors and certain regimes recorded there.

## literature

• A. Baumgartner, H.-J. Liebscher: Textbook of hydrology. Volume 1: General Hydrology. Stuttgart 1990.
• A. Bronstert (Ed.): Flow formation - process description and case studies. (= Forum for hydrology and water management. Issue 13). Hennef 2005.
• DIN, German Institute for Standardization e .V .: DIN 4049-3 Hydrology, Part 3: Terms for quantitative hydrology . Beuth-Verlag, Berlin 1994.
• C. Glugla, E. Müller: Groundwater recharge as a component of runoff formation. (= Freiburg writings on hydrology. 1). Freiburg i. Breisgau 1993.
• G. Glugla, E. Müller, P. Jankiewicz, C. Rachimow, K. Lojek: Development of methods for calculating long-term mean values ​​of area-differentiated runoff formation . Final report on the DFG project GI 242 / 1-2 "Wasserhaushaltsverfahren", BfG branch office Berlin 1999.
• G. Glugla, P. Jankiewicz, C. Rachimow, K. Lojek, K. Richter, G. Fürtig, P. Krahe: BAGLUVA water balance method for calculating long-term mean values ​​of actual evaporation and total runoff . BfG report 1342, Federal Institute for Hydrology; Berlin / Koblenz 2000.
• W. Struckmeier: Water balance and hydrological system analysis of the Münsterland basin. (= LWA series of publications by the State Office for Water and Waste NRW. Volume 45). Düsseldorf 2000.