2.13 Mean Annual Actual Evapotranspiration Depth

Precipitation and evaporation are closely interrelated factors in Orohydrography Soils Land Cover Figure 2 shows the HAD Maps and informations which were used the hydrologic balance. The water supplied by precipitation Map 1.1 Map 1.3 Map 1.4 in the procedure calculating actual evapotranspiration depths. This enables the evaporation process to take place, which is stimu- demonstrates the close connection between the key parameters and Average Duration of Snow Cover Mean Annual Potential Evaporation lated by the energy budget. In contrast to precipitation, this Depth as Grass Reference hydrometeorological factors. Evapotranspiration process is ongoing and is influenced by the daily rhythm (tem- The actual evapotranspiration depths were validated by comparing perature, radiation). The evaporated water can be transported Map 2.10 Map 2.12 the runoff calculated on the basis of (P corr - ETa) with runoff meas- by means of convection to higher atmospheric layers, where it Mean Corrected Mean Corrected ured in catchment areas (Map 3.5 “Mean Annual Runoff Depth”). initializes cloud formation, which are a precondition for pre- Annual Precipitation Depth Precipitation Depths of the Summer Half-Year cipitation generation. Thus, the evaporation process is neces- Map 2.5 Map 2.6 sary to maintain the water cycle. Map Structures Unlike precipitation, it takes a great deal of effort to measure evapotranspiration. Weighable lysimeters are one of the few Mean Actual Annual Evapotranspiration Depth Map 2.13 shows the mean annual actual evapotranspiration depth possibilities to determine the evapotranspiration depth for dif- Map 2.13 in the time series 1961–1990 in the form of a gridded structure 2 ferent types of soil and certain types of land use (e. g. crops) with a resolution of 1 km . The mean evapotranspiration depth for Fig. 2 Use of HAD-Maps to calculate the mean annual depth of Germany is thus 532 mm/a. The key is the same as for Map 2.12 constantly and as a daily value to a sufficiently precise degree actual evapotranspiration in mm/d and, above all, immediately. In forest areas, non- (grass reference evapotranspiration) with class amplitudes of 50 weighable lysimeters provide multi-year mean evapotrans- mm/a resp. 25 mm/a for the range 500 to 600 mm/a. This illustrates piration values as an annual total in mm/a. Due to their complexity, all of these measurements that the actual evapotranspiration depth varies more from location to location than the grass have to be restricted to just a few experiment stations. Indirect meteorological procedures, in- reference evapotranspiration. Apart from precipitation and the evapotranspiration required by cluding the calculation of the grass reference evapotranspiration (Map 2.12), are based on en- the atmosphere as expressed by the grass reference evapotranspiration, the actual ergy- and water-balance equations with the link between the meteorological parameters, which evapotranspiration depth is largely determined by soil and land-use factors, which vary much have to be measured directly, and the evaporative heat flux or water-vapour transport in the more. boundary layer taken into account. In order to calculate mean annual totals for the actual The actual evapotranspiration values are below 350 mm/a in urban areas and the higher loca- evapotranspiration depth throughout the country, the findings from the site have to be applied tions in the Alps, the Erzgebirge mountains and other upland regions, and above 700 mm/a to areas with similar characteristics. primarily in the Oberrheinebene (Upper Rhine plain) and in areas with a groundwater level Viewed over several years, the difference between the factors of precipitation (Map 2.5) and close to land surface. Due to their extensive impermeable areas with low water-storage evaporation determines the maximum volume of water available for water supply and distri- capacity, urban areas’ actual-evaporation values are low which means that, apart from the cit- bution, the total runoff (Map 3.5). ies of Berlin, Hamburg and Munich and the Ruhr district, smaller towns such as Rostock or Münster also record low actual evaporation. Lakes, on the other hand, stand out for their higher depth of evaporation. Methodology Figures 4 and 5 show west/east and north/south sections respectively for grid cells indicating the mean actual evapotranspiration depth; the sections cross at the summit of the Harz Moun- The Bagrov method has proven generally usable for determining the mean annual totals for tains. In line with the extreme variability of land cover, evapotranspiration fluctuates much the actual evapotranspiration depth in the climatic conditions prevalent in Germany (G LUGLA more significantly than precipitation (Fig. 1 and 2 in Map 2.2), i. e. the climatic differences & M ÜLLER 1997). This method combines mean annual values for the factors corrected pre- are of less importance than the influences which differ from region to region. The grass refer- cipitation P , maximum evapotranspiration based on land use ETmax and actual evapo- corr ence evapotranspiration curves reflect the climatic influences, which are modified by the land transpiration ETa (Fig. 1). cover and the soil. The modified BAGLUVA method, developed by G LUGLA et al. (2001) fulfils the boundary Impermeable areas can therefore al- conditions for arid and humid regions: 700 ways be identified by their minimal sandy soils 675 Q Q cohesive soils 626 ETa P corr , if Pcorr 0 limiting water availability 600 depths of actual evapotranspiration, 600 584 ETa Q ETmax, if P Q or ETmax Q 0 limiting energy availability 560 corr which can be clearly assigned to a 540 500 488 496 494 specific place. Due to their high de- 471 1 n=8 5 Bagrov-parameter n gree of sealing, industrial areas and 418 ETa 3 400 2 ETmax 1.6 sandy cohesive airfields have the lowest values. In

1 soils soils mm/a 300 0.7 forest areas, the grass reference Q nFK = 12 % Q nFK = 20 % 0.5 evapotranspiration is exceeded 228 impermeable 0.16 200 0.3 surface though the soil, altitude and type of 0,5 0.2 bare sites 0.53 1.02 forest determine the absolute value of 100 0.1 grassland 0.91 1.87 actual evapotranspiration. The above- Bagrov-relation 0 arable land 1.16 2.10 average actual evapotranspiration dETa ETa n 1   dP −=  ETmax  deciduous forest values in some of the grid cells point corr   1.94/2.21 2.04/2.22 50/100 years bare sites grassland to the presence of water sheets. impermeable arable land water bodies 0 P coniferous forest surface deciduousconiferous forest forest 0 1 2corr 3 2.00/3.76 2.01/3.77 ETmax 20/80 years Figure 3 shows the mean actual evapotranspiration values in Ger- Fig. 3 Actual evapotranspiration depth of vaious types of land cover on different soils Fig.1 Bagrov-relation (Bagrov 1953) many based on a combination of similar units of area. This clearly The Bagrov parameter n describes, in particular, the utilisation of the soil’s water-storage shows the considerable influence of properties and was quantified as a function of soil type and land-use type on the basis of ex- land cover on evapotranspiration, modified by the soil properties. In areas covered with veg- tensive lysimeter evaluations. The regression equations derived allow to calculate n using a etation, cohesive soils increase evapotranspiration by 20 to 30 mm/a. The low value for im- soil map (scale 1 : 1 000 000) showing also the different types of land use, and which served permeable areas illustrates the importance of considering the evapotranspiration when devis- as the basis for the Maps 4.1 to 4.4 and 1.3, plus the land cover data (Map 1.4). ing land-use plans. Additional land-use characteristics and soil properties can be considered taking into account the maximum evapotranspiration. For this purpose, the grass reference evapotranspiration 800 ET 0 in Map 2.12 is used as a reference in order to calculate the maximum evapotranspiration Weserbergland Rhine Haltern deciduous forest at Schwarze Elster 750 Hohe reservoir for various soil-covering categories (impermeable areas, areas without vegetation, grassland, Mark Egge Mts. Solling Western Harz Mts. biosphere refuge 700 Nieder- arable land, deciduous and coniferous forest) in the various climatic conditions when the wa- Borken- Mittlere Elbe Lausitzer berge Eastern Harz Mts lausitz ter supply is adequate: 650 Grenzwald 600 550 ETmax = f · f H · ET 0 (in mm/a) (1) 500 Factor f is also based on the soil properties modified by land use and stems from the evalua- 450 tion of multi-year measurements in Germany using lysimeters, where the water supply was 400 Elbe mm/a flats sufficient (e. g. deep soil from loess). Factor f indicates the modification of the evapo- 350 Salzder- H helden Mehr Langen Güsten transpiration due to the different slope inclinations and exposure as derived from the altitude 300 berg Clausthal- Bernburg/ Calau Forst Haltern data in Map 1.1 “Orohydrography”. 250 Zellerfeld Saale Ahlen Paderborn airfield Bad Lippspringe Cottbus 200 with Altenau Quedlin- Hohenbucko industrial area burg The actual evapotranspiration depth is composed proportionately for the units classified in industrial area Brocken 150 Hamminkeln Lüchtringen industry at railway in Wesertal Blankenburg Dessau-Bitterfeld Map 1.4 “Land Cover” on the basis of the evapotranspiration ETa calculated using the Bagrov 100 equation (Fig. 1) for the six above-mentioned classes (e. g. unit 1.4.1 “green urban areas” is 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 given as 50 % grassland and 50 % deciduous forest) and relates to snow-free periods. W longitude E Depending on the season, periods during which the areas are covered with snow contribute to Fig. 4 West/east section of mean annual values of actual evapotranspiration ( ) and grass reference evapotranspiration ( ) 1961–1990 at latitude 51°50‘ north the total evaporation to differing degrees. The parameters shown in maps A to D in Map 2.10

“Average Duration of Snow Cover” are used to estimate this period over the year in which the 800 Oerelen fen Steigerwald large fen Haßberge evaporation values given in Table 1 are assumed. It should be borne in mind that regional and 750 at Elbe-Seitenkanal Lake Eutin Elbe Thuringia Forest Donau-Ried local factors can cause the evaporation to differ from these values. 700 Lake Tankum South Harz Wierener North 650 Berge Harz 600 Table 1 Average evaporation from compact snow cover (R ACHNER 1999) 550 month Sep Oct Nov Dec Jan Feb Mar Apr May 500 snow evaporation in mm 0.20 0.10 0.05 0.00 0.05 0.15 0.30 0.30 0.30 450

mm/a 400 arable land Villenbach 350 Büchen Zell a. The evaporation depths of inland waters are calculated in accordance with a model invented Ober- Themar Wechingen Hankens- Main 300 mehler Arberg by R ICHTER (DVWK 1996), which combines a Dalton-type evaporation formula (Eq. 2) with Neetze büttel Langen- Walkenried Oberschönau Ansbach 250 Lübeck Calberlah neufnach a model for calculating the water-surface temperature in order to calculate the vapour-pressure Braunlage Neustadt/ 200 Braunschweig- Tambach/ Aisch Kaufbeuren gradient on the water surface. Weddel heathland area Dietharz 150 Eutin with of Brocken summit Schlüsselfeld industrial area Marktoberdorf EW = a · (e S(T W,O ) – e) + b (in mm/d) (2) 100 54.5 54.0 53.5 53.0 52.5 52.0 51.5 51.0 50.5 50.0 49.5 49.0 48.5 48.0 47.5 where E W is the evaporation loss from the waters, e S(T W,O ) the saturation vapour pressure at N latitude S temperature T W,O on the water surface and e the vapour pressure in the air. The values of coef- ficients a and b were calculated on the basis of multi-year series of observation data on the Fig. 5 North/south section of mean annual values of actual evapotranspiration ( ) and grass reference evapotranspiration ( ) 1961–1990 at longitude 10°10‘ east evaporation depth for various types of waters. The water-surface temperature resp. evaporation depth depend on the meteorological condi- tions and, furthermore, waters-specific parameters, such as depth and wind exposure as well as, in some cases, anthropogenic thermal load. The mean annual values for evaporation of in- land waters given in Map 2.13 were calculated based on the simplified assumption of a mean waters depth of 6 m and a sufficiently ventilated water surface.

For arable areas on sandy soil, e. g. the irrigated areas east of Magdeburg (A CHTNICH 1980), field irrigation is assumed dependent on the summer precipitation and the summer/annual pre- cipitation ratio.