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4.2 to 4.4 Soil-Water Retention Parameters

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4.2 to 4.4 Soil-Water Retention Parameters 4.2 to 4.4 Soil-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 percolation proper- contained tables permitting the field capacity, available water capacity, air capacity, and total ties and thus determines pore space to be estimated from soil texture class, bulk density, and humus content. Since soil moisture amongst other things the not available Pleistocene sands and loams 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) clay 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 silt 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- sand 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 capillary 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- water content (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 soils 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 Soil water retention parameters given in storage capacity for groundwater 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 wilting 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 drainage 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.
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