The Soil Moisture Regime Estimated by Means of Climatic Data
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The Arab Center for the First Training Course in Agrometeorology for Arid Zones Studies of Arid Zones Damascus, h4ay and Dry Lands (A C S A D) IO - 23 I082 The Soil Moisture Regime estimated by means of Climatic Data. Its relationships with Annual Rainfall and .._ with Bioclimatic Classifications under Mediterranean Climatic Conditions. Paul BILLA.. Pedologi.st of the Oversea Scientific and technical research Institute (O R S T O hl. F'rance) .- . Dans de nombreux pays les mesures du régime hydrique des sols sont rares et son estimation au moyen des données clinstiques ne peut se faire que pour un nombre limité de stations météorologiques. 11 est alors malaisé de délimiter avec une approximation suffisante les régions ayant un régime hydrique donné. C'est là une difficult6 pour l'utilisation de la Taxonomie américaine des sols. Le texte rappelle d'abord la définition des régimes hydriques selon la Taxonomie américaine et le principe de leur estimation au moyen des données climatiques. Les paragraphes suivants exposent les relations qui existent, dans quatre pays arabes B climat de type méditerranéen, entre les classes de régime hydrique estimé et a) la pluviométrie moyenne annuel- le, b) les subdivisions bioclimatiques des systèmes d'Emberger et de Ragnouls et Gaussen. De telles relntións permettent, povr un pays donné, d'utiliser les cartes pluviométriques et bioclimntiques dans le but d'méliorer le tracé des limites entre les régions 8 régimes hydriqaes différents. ABSTRACT In many countries measured data concerning the Soil hloisture Regime (ShfR) are scarce; furthermore the ShfR estimation by means of climntic data is possible only for a somewhat limited number of meteo- rologiesl stations. As a consequence it is not easy to draw boundary lines between areas mith different SMR. This is a difficulty freqnent- ly met with when using the U.S. Soil Taxonomy. In a first part this pnper shortly recalls the definition of the Soil Moisture Regimes according to Soil Tmonomy and states the prin- ciples of their estimation by means of climatic data. ' The following paragraphs deal with the relationships found, for some arsb countries with mediterranean climate, between the classes of estimated ShlR and a) the average annual precipitstion, b) the bioclimste according to the bioclimatic classifications of Einberger and of Bagnoulls and Gaussen. Such relationships allow to use, in a given country, Annual pre- cipitation maps and bioclimatic maps in order to mark with R better accuracy the boundaries between regions with diff ererrt ShR. 6 hilots-cl6s: Régime hydrique des sols. Estimation par donn6es climn,ti- ques. Relations avec Pluviométrie annuelle et, avec Bioclim3ts m6di terrmnéens ., CONTENT , Page I. Introduction I' 2. The Soil Moisture Regimes according to "Soil Taxonomi)."; definitions I 2.1. Soil Moisture Control Section I 2.2. Moisture and Dryness 2 2.3. Soil Temperature Criteria 2 2.4. Classes of Soil Moisture Regime 2 2.5. Remarks 3. Estimation of the Soil Moisture Regime by means of Climatic data 3 3.1. Principle of estimation of the moist and dry periods in the moisture control section 3 3.2. Newhall's method 4. 3.3. Graphic method 5 3.4. Soil temperature estimation 5 3.5. Results of SMR estimation. Remarks 5 4. Relationships between estimated Soil Moisture Regime and average annual Precipitation 6 1 I 4.1. Estimated SMR and mean annual Precipitation . 1 and Air Temperature 7 4.2. Estimated Sm, "dry in all parts" period of the moisture control section and mean annual precipitation 7 4.3. Comparison of the relationships found for Syria, Lebanon, North Morocco and Tunisia 8 5. Tentative relationships between estimated SMRt-and Bioclimatic classifications 8 5. I. Estimated SMFt and Mediterranean Bioclimatic Stages of Emberger 8 5.2. Estimated SMR and Mediterranean Bioclimates of Bagnouls and Gaussen 9 6. Some conclusions 9- References 10 Figures. Annexe. The Soil Moisture Regime estimated by means of Climatic data. Its relationships with Annual Rainfall and with Bioclimatic Classifications under Mediterranean Climatic conditions. I 0 INTRODUCTION The Soil Moisture Regime is the succession of the moisture con- ditions in the various soil layers during the successive seasons of the year. Likewise the Soil TemperRture Regime is the succession of the temperature level in the soil layers throughout the year. The knowledge of these regimes is important: ci - to understand and explain the pedogenesis, i.e. the development (past and present) of the soil physical and chemicel properties; - to achieve R successful management of farming in rainfed areas. Water and heat are most impor$nnt factors for the chemical, physico- chemical and biological activities. Therefore it seems normal for B soil Classification to use, at any level, some criterin related to the Soil Moistlire and Temperatura Regimes. Most soil classifications include such criteria, one way or another. The French classificatim (1967) define the subclasses (second higher level) by reference to the "pedoclimatic" conditions, a synthesis of the soil regimes for moisture, temperature, red - ox conditions and soil solution concentration. The Canadian clrtssif ica - tion (1978) introduce the soil moisture regime at b lower class (soil family); et cetera ... I However it is the recent U.S. Soil Taxonomy (1975) which uses the present Soil Moisture and Temperature Regimes in the most systematic way, at high classification levels and with precise quantitative cri- teria. In this paper it rill be dealt only with the Soil Moisture Regime (SMR) as defined in the U.S.%oil Taxonomy". 2. THE SOIL MOISTURE REGIMES ACCORDING TO "SOIL TAXONOMY" DEFINITIONS. The SMR are defined by the length of moist and dry periods in a soil control section and, in addition, by some soil temperature crite- ria. 2.1. Soil Moisture Control Section (MCS). "The upper boundary of the MCS is the depth to which B dry (water tension > I5 bars but not air dry) soil rill be moistened by 25 mnr of water within 24 hours. The lower boundary is the depth to which a dry soil will be moistened by 75 mm of water within 48 hours. These depths are exclusive of the depth of moistening along any cracks or animal burrows that are open to the surf ace". The depth and thickness of MCS depend on the soil physical charac- , teristics which determine the soil Available Water Capacity (AWC). As a rough approximation the depth of the upper and lower boundaries are: -in fine loamy, silty, clayey soils: IO - 30 cm; - in coarse loamy soils: 20 - 60 cm; - in sandy soils: 30 - 90 cm. 2 If the soil is shallow over R hard rock or indurated layer, the ' lower boundary of the MC9'is the upper limit of the hard material. In this case it needs less than 75 mm of water to moisten the soil down to the lower boundary. 2.2. Moisture and Dryness. The soil is "moist" if it holds water at a tension 4 I5 bars (available waxer). This implies that the water con- tent of the"moist" soil may vary widely, between saturation and the wilting point. The soil is rldrylf if it holds only water a't a tension 3 I5 bars.' (Suah high tension may occur in moist but very salty soile, the soil is then considered physiologically dry.) The MCS may be %oist in all parts" (i.e. throughout its whole depth) or "dry in all parts", or"moist in some part" (which is equiva- lent to "dry in some part"). The lenght of the moist and dry periods of the MCS is measured by a number of either consecutive or cumulative drtys. Under mediterra- nean climatic conditions the moist period as well as the dry period are normally composed of consecutive drtys. 1 2.3? Soil Temperature Criteria. These are: - ,The Idean Annual Soil Temperature (WT); It may be obtained by mea- ' suremenkat a minimum depth of 50 cm made at regular time intervals throughout the year. - The Difference (DIF) between the mean summer and winter soil tempe- raturesat 50 cm depth (or at a lithic or paralithic contact).* - The periods during which the soil temperature at 50 cm depth (ST 50) is either > SOC or between 8 and 5OC or <5OC. They may be deduced . from the curve of the mean monthly soil temperature at 50 cm depth. 2.4. Classes of Soil hfoisture Regime. The criteria of definition for the SMR classes (except Aquic) are put in a comparative way in Table I. - Aridic (= Torric) : the moisture regime of arid regions and part of the semi-arid regions; in-st years rainfed cultivation is not pos- aible; there is no leaching; soluble salts accumulate. - Xeric: the typic moisture regime of regions with mediterranean cli- mate, i.e. cool or cold rainy winter, warm and dry summer. The rainfall, coming ip winte&hen evapotranspiration is minipun, is particularly effective for leaching. - Udic: occurs in regions of humid climste (temperate or tropical) with well distributed precipitation throughout the year. Water moves down through the soil at least at some time. *MAST and DIF are also used in the U.S. Soil Taxonoqy to define the Soi 1 Temperature Regime (STR) ; for instance : MAST = 0-8OC 8-15OC 15-22OC > 22oc with DIF>5OC, STR: frigid mesic thermic hyperthermic with DIF< 5OC, STR: isofrigid isomesic isothermic isohyperthermic .I Table-t - Definition of the soil moisture reqime . classes ! (except Aquic) according ta Soil Taxonomy i -4 Gonsecutiye days 4 C umulat iv e time 1 / A Seasonal conditions ~ Others conditions Part of the time Others conditions when ST-.> 5@c (days) -5 I t $ [ !4months I -4monUls i I I I I f I I when 5TS,)8*C 1 I* I AR1 O I I IC I l I I moi&/MOIST < 90 piK-[ (=Tsrr¡c) 1 I I I I (most (most years 1 I I years) I I 1 I 1 I I f I f I I MAST 222.C I I .jdry/DRY)SOI or l II I (most years) I I I DIF <SC I I I I I I I I I USTIC I If I I I I MAST < 22.C if---+- --:- and 1-1 m’ DIF 3 5.C I >/ 61ycars/ IO) I (most years) i I I I I when ST,, > 8.C i MAST C 22% F XERIC i í I l I f if I I I -i UDlC I (most years) I i 4 ’ Maisture control section MOIST, : moist in alt parts dry : dry.