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Home , Field capacity, Soil moisture

4.2 to 4.4 -Water Retention Parameters

hPa pF The moisture regime of a Pedo-transfer functions for determining the water-retention parameters 7 soil is a function of its stor- -10 7 The old (3rd) edition of the German soil-mapping guidelines (KA 3; AG B ODENKUNDE 1982) age and proper- contained tables permitting the , available water capacity, air capacity, and total ties and thus determines pore space to be estimated from class, bulk density, and humus content. Since amongst other things the not available Pleistocene and were markedly underrepresented in the previous database, the 5 to plants quality of the soil as habitat -10 5 estimated values given in KA 3 were again checked on the basis of a large database (H EN - PWP for plants. The water input 4,2 NINGS & M ÜLLER 1993). Evaluation of about 1700 pF curves in the laboratory database of the into the soil from precipita- Lower Saxony soil information system (Niedersächsisches Bodeninformationssystem, NIBIS) tion is only partly free to soil moisture showed that use of the tables in KA 3 led on average to an over-estimation of the total volume. matric potential 3 available to plants -10 3 move and migrates down- (nFK) In addition, the humus-dependent positive and negative corrections to the water-retention pa- wards under gravity as per- rameters were significantly underestimated. FK colating water; a certain pro- 1,8 portion of it is retained as Since the 4th edition of the German soil mapping guidelines (KA 4; AG B ODEN 1994) con- 1 -10 1 and adsorbed water. tains a new soil texture class triangle with 31 partially newly defined soil texture classes (Map The forces exerted by the 4.1, Fig. 2), the tables used to derive soil physical parameters had also to be recalculated. For -1 0 solid matter in the soil are this purpose, over 6000 datasets of existing measured values from eight of the German Fed- 0 20 40 60 described according to the eral States were placed into a central database and evaluated for calculation of new pedo-trans- (vol %) potential concept in terms of fer functions (K RAHMER et al. 1995). As a result, we have new, representative, statistically Fig. 1 pF-curve as relation between matric potential and soil the soil-water matric poten- sound estimates of LK, FK, nFK and saturated hydraulic conductivity as functions of soil tex- moisture content for typical sandy, silty, and clay tial and measured in hPa. ture class, dry bulk density and humus content, which have been incorporated unchanged into The relationship between log KA 4, the DVWK guidelines on water management, as well as into the German standards pub- of the soil-water matric po- lication DIN 4220-1. tential and the moisture content in wt % or vol % is called water retention curve or pF curve. In the international context, there are further pedo-transfer functions available, which are It is often used to characterise the influence of soil properties on the soil moisture regime. partly based on other physico-empirical approaches and which incorporate other input vari- Three different water-retention parameters can be derived from the water retention curve ables. T IETJE & T APKENHINRICHS (1992) and T IETJE & H ENNINGS (1993) classified and (Fig. 1). These are defined by the different moisture contents for different soil-water matric assessed several of the existing approaches. At the European level, the available measurement potential intervals, as follows: values of soil physical parameters were placed in a Europe-wide database HYPRES (hydrau- lic properties of European soils), in order to use them to derive further pedo-transfer functions Field capacity (FK) pF > 1.8 The amount of water that the soil can hold against for the most important European parent material and horizon groups (W ÖSTEN et al. 1999). gravity All three water-retention parameters shown on Maps 4.2, 4.3 and 4.4 were estimated using the Available water capacity (nFK) pF 1.8 - 4.2 Field capacity minus the water held in the fine pores; table in the soil mapping guidelines mentioned above. First a basic value is determined from proportion of the field capacity available to plants Table 1, which is valid for mineral Air capacity (LK) pF < 1.8 Air content at field capacity, which corresponds to the soils with up to 1 % organic matter. Table 1 retention parameters given in storage capacity for and/or perched This value is then modified by mm/dm horizon thickness depending on water positive or negative corrections de- soil texture class and class of bulk density pending on the humus content. In Field capacity is conventionally quoted in Germany as moisture content in vol % at a soil- the case of complete soil profiles, soil texture nFK LK FK water matric potential of pF = 1.8 (equivalent to mm head of water per dm soil thickness) and class pore-Ø 0.2 - 50 µm pore-Ø > 50 µm pore-Ø < 50 µm the parameters are determined for pF 4.2 - 1.8 pF < 1.8 pF > 1.8 in some other countries at pF = 2.5. The permanent point, conventionally quoted as the effective rooting depth by sum- kt-class kt-class kt-class matric potential of pF = 4.2, is the limit at which crops begin to wilt irreversibly. The avail- ming the values for every horizon. 1+2 3 4+5 1+2 3 4+5 1+2 3 4+5 able water capacity is defined by two soil-water matric potentials: above pF = 4.2 all the mois- Ss 15.5 11 10.5 24 21.5 18.5 22 15.5 14.5 Sl2 19.5 17.5 17 17 14.5 10.5 28 24.5 22 ture is held in the fine pores and is not available for plants, and below pF = 1.8 the moisture is The concept of an estimated water Sl3 22.5 18.5 16 12.5 11.5 7.5 34.5 27 24 retention curve derived from sta- Sl4 20.5 17.5 14.5 11.5 10 6 34 29 25.5 relatively readily removed from the pores (air capacity). The sum of the field capacity and the Slu 27 21.5 18.5 8 7 5.5 41.5 32 29 air capacity is the total pore volume of the soil. ble, time-constant soil properties St2 20 15.5 11.5 18.5 17 13 28.5 22.5 20 St3 17 14.5 12 12 9.5 7.5 33 29 26 suggests that the parameters shown Su2 19.5 16.5 14.5 18 15.5 12.5 27.5 22 20 The water-retention parameters of a soil depend on pore volume and pore size distribution and on the maps, such as the field ca- Su3 24 22 19 12 9.5 8 32.5 28.5 25.5 therefore vary with type of soil. Figure 1 shows typical water retention curves for a sand, a silt Su4 26.5 24.5 21.5 10.5 8 5 35 31 28.5 pacity, are quasi-constant soil pa- Uu 28 25.5 23 7.5 5 2 41 36.5 35 and a clay. It can be seen that the sand is characterised by a high proportion of loosely bound rameters that correspond to an of- Us 28.5 26 22 8.5 6.5 3.5 41 34 30 water and a high air capacity. The different shapes of the characteristic curves for the two other Uls 26 22.5 20.5 8.5 7 3.5 40.5 33.5 31.5 ten recurring hydraulic equilibrium Ut2 27.5 25.5 23 9.5 5 2 40 36 34.5 types of soil is caused by their different pore size distributions: medium size pores dominate Ut3 25.5 24 22 10 4.5 2 38 36 34.5 situation. Particularly in the case of Ut4 22 20.5 17.5 9.5 5 2 38 36 33.5 in the silt, and fine pores in the clay, which tends to hold the moisture tightly. In comparison, a deep water table, the soil is so Lu 19.5 16 14 6.5 5.5 4 42 35.5 32 the clay has the higher field capacity and the silt the higher available water capacity. Apart Ls2 20 14.5 13 9 7 5.5 39.5 32.5 29 thoroughly drained that the equilib- Ls3 19.5 15 12.5 7.5 6.5 4.5 41.5 32.5 28.5 from the soil texture class, the bulk density, humus content and structure affect the capacity rium situation is not attained owing Ls4 19 15.5 12 8.5 7.5 5.5 40 32 27.5 of the soil to hold water. Lts 15 12 10 4 3.5 3 45 36.5 30.5 to the low hydraulic conductivity, Lt2 16.5 13 10.5 5.5 4.5 3 44.5 36.5 31.5 Lt3 14.5 10.5 8 4 3 2.5 45 38.5 33 as especially in clays. The influ- Tt 16 11.5 8 3 2.5 1.5 54 42 39.5 ence of hysteresis, i.e. different Tl 14 9 6.5 4 3 2.5 50 40 34.5 Map Structures Ts2 15.5 10 7.5 5 4 2 47.5 38.5 35 water retention curves correspond- Ts3 16 10.5 8 9 7 3.5 42 34 31.5 Ts4 16 10.5 8 11 8.5 4.5 37.5 30.5 29.5 The soil water retention parameters allow us to draw conclusions about how tightly water is ing to wetting and condi- Tu2 15 10 7.5 3 2.5 2 50 40.5 35.5 tions, cannot always be reflected in Tu3 15.5 11 6.5 4.5 4 3 45 37.5 33 held and how much is stored in the soil, the velocity with which water is released from the soil Tu4 17 15 12 7 5 2.5 41 36 33.5 and the availability of the water for plants. They are shown in Maps 4.2. to 4.4: a mean water retention curve de- rived from pedo-transfer functions.  4.2 Field capacity up to 1 m soil profile depth (FK ) 10dm The values on Maps 4.2, 4.3 and 4.4 are based on pedo-transfer functions, which in turn are  derived from soil texture class, bulk density and humus content, but do not take the soil struc- 4.3 Available water capacity in the effective zone (nFK We ) ture into consideration.  4.4 Air capacity in the effective root zone (LK ) We The accuracy of the estimate obtained using pedo-transfer functions can be assessed with dif- To characterise a complete soil profile, the available water capacity and the air capacity are ferent measures of error which quantify the deviation of the measured and estimated water re- usually summed over the effective rooting depth (Map 4.1). In addition, the field capacity is a tention curves. Even the relatively “best” method produces a mean deviation (over a wide measure of the influence of the soil on water and solute transport conditions and is not range of soil texture class) of measured moisture contents of about 3-4 vol % (T IETJE & summed over the effective rooting depth corresponding to the type of soil, but to a fixed depth HENNINGS 1993). In addition, the quality of the estimate varies as a function of soil texture class: all pedo-transfer functions valued as “best” show minimal errors for the clayey to interval of 1 m (FK 10dm ). If this were not so, then silty and clayey soils would be assessed the same and would appear undifferentiated in the corresponding map. silty clays, increasing deviation of the estimated from the measured results for the loams and the poorest estimate for almost clay- and silt-free sands. Map 4.2, which is not subdivided according to land-use, was compiled on the basis of Almost all the pedo-transfer functions in the literature were validated using water retention Map 1.3. The field capacity FK 10dm refers to a standard profile depth of 1 m. The classes of the curves, which were derived from measurements made on core samples under laboratory con- parameter FK 10dm are derived from a frequency distribution of all values regardless of whether they refer to agricultural or forest land. The field capacity classes are differentiated by colours ditions on ceramic plates. It is not yet certain whether results obtained with this method are from pale yellow for very low to dark brown for very high. also valid under field conditions. Validation based solely on the measured moisture contents does not answer the question as to which pedo-transfer function is the most suitable to use for The polygon pattern of Maps 4.3 and 4.4 is the same as that of the land-use of Ger- simulation models of the soil moisture regime. Future work should concentrate on solving this many (see notes on Map 4.1 “Depth of the effective root zone”), i.e. the result of superimpos- problem. ing the 1 : 1,000,000 Soil Map of Germany (BÜK 1000) (H ARTWICH et al. 1995) and a map showing the current pattern (CORINE Land Cover, S TATISTISCHES B UNDESAMT 1997), generalised to the scale of 1 : 2,000,000. The available water capacities nFK and air Practical Information capacities LK of the individual horizons are both summed over the effective rooting depth.

The resulting nFK We and LK We values are then classified separately for the two types of land The quality of the data on which Maps 4.2, 4.3 and 4.4 are based, i.e. data from reference pro- use, since the effective rooting depth is a function of the soil water requirements of the specific files of selected dominant soils of the soil associations in the 1 : 1,000,000 Soil Map of Ger- plants and is assessed separately for agricultural and forest sites on the basis of separate algo- many or BÜK 1000 has already been discussed in the explanatory notes to Map 1.3. The val- rithms, i.e. is increased by 20 % on average for forest sites. ues given on the maps (derived only from dominant soils) may show some divergence from In Maps 4.3 and 4.4, both the effective rooting depth and the water-retention parameters of the mean water-retention parameters calculated according to the proportions of the areas each individual horizon have a spatially differentiating effect. This approach subdivides the occupied by the dominant soils and all associated soils. maps according to soil conditions and the dominant type of land use into two hierarchically In principle, the parameters such as nFK We can be used as input data for simple functional classified spatial patterns. Areas that have not been assessed, about 3.2 % of the area of Ger- models to simulate the soil moisture regime or for empirical nomograms and regression equa- many, are given a special symbol. Both nFK and LK of the arable and grassland soils, like We We tions for determining of the mean annual percolation rate. The same is true of FK 10dm as a the parameter effective rooting depth described above, are represented by a colour scale from measure of the retention capacity of inorganic contaminants not absorbed by the soil matrix, pale yellow (very low) to dark brown (very high), and those of the forests are pale green (very with whose help, for example, the potential hazard of nitrate can be assessed, as long low) to dark green (very high). The same colour scale, the same classes and the same values as information on the climate is also taken into account. have been chosen to facilitate comparison of the maps.

The highest values of the available water capacity nFK We on a national scale are characteristic of the soils of the landscapes under both agriculture and forest. In this case the effects of We and nFK are additive. The highest FK 10dm values lie on the agriculturally used fen and raised-bog soils and on clay-rich -plain soils. The FK 10dm of the loess soils is insignifi- cantly smaller. The lowest values of both these parameters are those of the soil associations of the mountainous and hilly areas, where shallow rooting depths as well as a large propor- tion of rock fragments in the tend to limit the two parameters. The air capacity shows a complementary spatial pattern to that of the field capacity: The maximum air capacities occur in soils in forests developed from glaciofluvial and fluvial sand in the old and young moraine landscapes and in the Pleistocene lowlands. Since these sites have soils that show the shallow- est rooting depths, the range of possible LK We values is limited, especially in agricultural areas.