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Evapotranspiration changes in the forest-steppe and steppe zones under soil mulching

Evapotranspiration changes in the forest-steppe and steppe zones under soil mulching This paper contains the method and results of calculation evapotranspiration and its structure ­ transpiration and evaporation from traditionally tilled and mulched soil. The data considered indicate that in the arid regions of territory under study the evapotranspiration when soil is mulched does not change significantly and only the relation between the transpiration and soil evaporation changes. In the wet regions, evapotranspiration decreases when the soil is mulched which can result in a certain disturbance of the natural structure of the hydrological cycle and possible overmoistening soils. Keywords: Evapotranspiration; Evapotranspiration structure; Traditional agricultural practice; Soil mulching. INTRODUCTION The abundance of light and heat together with high soil productivity promoted the transformation of the southern Russian regions into an agricultural landscape. But these regions are characterized by insufficient precipitations (Shumova, 2001), which led to a wide-spread irrigation. The economic activities resulted in the local land overmoistening and salinization leading to the degradation of high-productive chernozem soils and removal of land from the agricultural use. Recently, in the agricultural production, the interest is paid to the nature friendly agricultural practice associated with the minimum man impact on the environment (Agricultural ecosystems, 1987). Natural ecosystems have on the soil surface a layer of felt and a litter of died-of plant mass, which allows them to optimally use the biosphere resources. Using the technology of a direct seeding and minimal soil treatment (Allen, 1985) with soil mulching as one of its links, the agroecosystem can be brought to a more natural form. The soil mulching promotes a substantial decrease in soil evaporation and an increase in transpiration, which results in the enhanced water supply of agricultural crops (Shumova, 1997). If under conditions of traditional agricultural practice in the forest-steppe and steppe zones part of transpiration to evapotranspiration in the spring wheat field amounts to 30­40% (Shumova, 2000) then in case of soil mulching with the straw it can amount to more than 60% (Shumova, 1997). However, a question is whether the magnitude of the evapotranspiration under soil mulching is changing or the structure of evapotranspiration changes. Presently, the problem is vital, as the reduced evapotranspiration in case of the soil mulching can result in the changed structure of a hydrological cycle: in the water accumulation in the soil, which is undesirable under conditions of the modern hydromorphism (Novikova and Nazarenko, 2007). Theoretical basis of the calculation method used in this paper is presented in papers of Budagovskyi and Novak (2011a,b). The objective of this work is to estimate the influence of soil mulching on the spring wheat field evapotranspiration in the foreststeppe and steppe zones. The problem is based on the numerical experiment, which to the evapotranspiration calculation under conditions of a traditional agricultural practice and soil mulching. The approach used in this work allows accounting for quantitative demands to the evapotranspiration of agricultural crops during their development. METHOD The evapotranspiration E in a general case includes the soil evaporation Es and the transpiration Et (Budagovskiy, 1964; Shumova, 2003) E = Es + Et . (1) The empirical formulas (Budagovskiy, Shumova, 1976) are used for calculating soil evaporation Es = Es0 Ve + 1- e ), (2) (3) (4) Es0 = b1s Ds d + b2 Re-s - B , s = e-1.1 , Ds = 0.8u u 1/2 + 0.7 (5) b1 = 0.7 , 1+ 1.56 b2 = 17.1T 0.026 , 1+ 1.56 (6) ( 235 + T ) e 235+T , 2 (7) where Es0 is potential soil evaporation (mm day-1), is empirical parameter dependent on hydrophysical soil properties (mm-1), V is available soil water storage of soil root layer (mm), P is precipitation (mm day-1), b1 and b2 are function of air temperature, s is function of the leaf area index, Ds is function of wind speed, d is air humidity deficit at 2 m (mb), R is net radiation (cal cm-2 day-1), s is coefficient dependent on the geographical latitude and season of year (Budagovskiy, 1964), is leaf area index (cm2 cm-2), B is heat flux in the soil (cal cm-2 day-1), u is wind speed at 2 m (m s-1), is the slope of the saturated water vapor pressure as a function of air temperature (mb OC-1), T is air temperature at 2 m (OC). Transpiration is calculated as follows E t0 = V E t0 Vcr k = 1- e )E s0 -P, (17) at at V Vcr V <Vcr , (8) m= 1 b D d + b2 R 1- 2e-s - Vcr 1 1 - P/ Es0 - 1 - 2 1- e Es0 + b2 B + Es0e . )( (18) Et0 = b1D 1d + b2 R 1- 2e-s - 1- 2 B - 1- 2 Es , (9) D = c1u c2u1/2 + 1 u = u + 0.4 , (10) 1 = 1- e- 2 = e-0.25 , (11) where Et0 is potential transpiration (mm day-1), Vcr is critical The calculation is performed with a ten day time step and covers a frost-free period (a period between a complete snow melting in spring and appearance of air temperature below zero in autumn). First, soil water storage then, soil evaporation and transpiration are calculated. A detailed assessment of the accuracy of the calculation scheme described is given in Shumova (2003). In case of the soil mulching, the scheme of the evapotranspiration calculation given above is used too. The differences are only in the use of other relations describing the soil evaporation. The estimation of the soil evaporation in case of mulching is based on the formula describing evaporation through a driedup soil layer (Budagovskyi, 1964), which for the mulching conditions is written (Budagovskiy and Grigorieva, 1991): available soil water storage (mm), D and u are function of wind speed, 1 and 2 are functions of the leaf area index, Es(m) = c1 = 2.4 , c2 = 4 m 1/2 -1/2 are coefficients determined from the Es0 , 1+ z m (19) field experiments. Critical available soil water storage in a one meter soil layer are described with the relation where Es(m) is soil evaporation under soil mulching, is parameter depending on diffusivity of water vapour in the mulch layer and on its heat conductivity, z m is thickness of the mulch layer. It should be taken into account that after each rainfall the mulch itself also retains some precipitation amount. On that basis two stages of the soil evaporation are considered in the presence of the mulch. The first stage is evaporation resulted Vcr = 60 + 4.2E0 , (12) (13) E0 = b1D 1d + b2 ( R - B ) , where E0 is potential evapotranspiration (mm day-1). Available soil water storage is determined according to the formula (Budagovskiy and Shumova, 1976) Ve = Vb + k / m e-m - k / m , (14) where Ve and Vb are available soil water storage (mm), at the beginning and at the end of the calculation time interval, respectively, is time interval (day). If the available soil water storage is above or equal to its critical values V Vcr , calculated in accordance with (12) and (13), k and m are calculated with the relations k = b1D 1d + b2 R 1- 2e-s - 1- 2 B + + 2 1- e )E s0 - P, (15) m = 2 Es0e (16) In the case of the available soil water storage is below its critical value V < Vcr , k and m are calculated with the formulas Fig. 1. Location of agrometeorological stations (AS). Dotted lines denote the boundaries of the forest-steppe and steppe zones (Berg, 1947; Berg, 1952). Circles denote agrometeorological stations with average annual data used for calculations; Full circles denote the AS annual data; 1 ­ Kazan, 2 ­ Mikhailov, 3 ­ Samara, 4 ­ Bezenchuk, 5 ­ Orenburg, 6 ­ Glukhov, 7 ­ Saratov, 8 ­ Nizhnedevitsk, 9 ­ Ershov, 10 ­ Uralsk, 11 ­ Kamennaya Step, 12 ­ VladimirVolynskiy, 13 ­ Mironovka, 14 ­ Poltava, 15 ­ Dzhanybek, 16 ­ Belovodsk, 17 ­ Novaya Ushitsa, 18 ­ Kirovograd, 19 ­ Konstantinovskiy, 20 ­ Kharabali, 21 ­ Mariupol, 22 ­ Kishinev, 23 ­ Kherson, 24 ­ Gigant, 25 ­ Odessa, 26 ­ Sarata, 27 ­ Krasnodar, 28 ­ Zolotushka. N. Shumova Fig. 2. Average annual values (a) and annual values (b) of the spring wheat field evapotranspiration in the case of the traditional agricultural practice E and from mulched soil Em . from the mulch drying, and the second stage is soil evaporation through the mulch layer. The first stage, the evaporation of the precipitation retained by the mulch is described with the formula (Budagovskiy and Grigorieva, 1991) Es(m) dt = E t0(m) E =V t(m) E t0(m) V cr at at V Vcr V 0.1, 0.5, 1.0, 5.0, 10.0, 20.0, and 30.0 mm. In this case, the result of calculations are monthly values of the soil evaporation; the ten day values are determined from the monthly values proportionally to the ten day values Es0 . When using the straw mulch (as the most attractive from the ecological viewpoint), according to the numerical experiments (Dzhogan nad Gusev, 2003; Gusev and Dzhogan, 2000), the thickness of its layer noticeably influences the soil evaporation only up to the thickness 4­5 cm. Thus, a 4­5 cm straw mulch layer (which corresponds to a mass of 10 t ha-1) is optimal that practically results in the maximally possible reduction in the soil evaporation. The technique of calculations of the evapotranspiration and soil water storage in the presence of the mulch is the following. The soil evaporation Es(m) , as was shown above, is determined independently from the precipitation and potential soil evaporation Es0 , which is determined using Eq. (3). The transpiration where m is parameter that depends on the mulch ability to Et0(m) = b1D 1d + b2 R 1- 2e-s - - 1- 2 Es(m) . - 1- 2 B (22) The available soil water storage under conditions of the sufficient water content V Vcr can be determined from the water balance relation Ve = Vb + (P - Es(m) - Et0(m) ) . (23) Fig. 3. Average annual values Em / E . Full circles denote the cases when Em = E . Et(m) is determined from the relation similar to (8), which in case of mulching becomes as follows When available soil water storage is below critical values V < Vcr , available soil water storage is determined by formula (14) at Fig. 4. Spring wheat field annual evapotranspiration for a frost-free period in the case of the traditional agricultural practice E (circles) and soil mulching Em (full circles). Table 1. Interannual variability of the spring wheat field evapotranspiration under a traditional agricultural practice and soil mulching. Station number 4 9 11 13 24 25 cients. Station Bezenchuk Ershov Kamennaya Step Mironovka Gigant Odessa Traditional agricultural practice E (mm) CV ( E) E (mm) 340 305 388 456 404 387 Soil mulching E (mm) E m (mm) m 334 308 363 374 396 376 37 51 39 14 26 29 CV ( E m) Note: E and E m are average values, E and E are standard deviations, CV ( E) and CV ( E ) are variation coeffim k = Es(m) - P , m = Et0(m) / Vcr . (24) (25) Evapotranspiration, as in case of the traditional agricultural practice, is calculated with a ten day time step and covers a frost-free period. The evapotranspiration calculations are based on standard observations of agrometeorological and actinometrical stations. The leaf area index is determined according to Shumova, (2003). Under conditions of the traditional agricultural practice (in the absence of mulching layer), the soil water storage values measured at the agrometeorological stations are taken in all cases as initial conditions. In the presence of the mulch the selection of the initial soil water storage that start the calculation has the following characteristic features. Selection of the initial soil water storage Vb , in the first ten day after complete melting of the snow cover in spring, should correspond to the same value as under conditions of the tradi- N. Shumova tional agricultural practice. When performing the calculations for several years, in the first year, the first measured soil water storage is taken as the initial water storage Vb . To determine the initial soil water storage in the next year, the spring soil water content, observed under traditional agricultural practice, is added to the final soil water storage Ve , which are observed in the last ten days with air temperature above zero in the previous autumn tion of the above scheme can be used for detail study of the water balance formation in the root layer in case of mulching (Gusev and Dzhogan, 2000); the scheme provides a detail description of physical processes in mulch layer and in the soil under the mulch in the frost-free period and for estimating the influence of a straw mulch on the soil water replenishment in spring to use a complex of models of the soil water formation in the winter-springtime as suggested in Gusev (1993). RESULTS AND DISCUSSION The average annual values of the spring wheat field evapotranspiration were calculated by the above relations, using the data from 28 agrometeorological stations (Fig. 1); for six of these stations (Bezenchuk, Ershov, Kamennaya Step, Mironovka, Gigant, and Odessa) the evapotranspiration was calculated for individual years. The calculations are performed in two variants: in the absence of the mulching cover and in the presence of a 5-cm layer of the straw mulch. Fig. 2a) shows relations between average annual values of the spring wheat field evapotranspiration for a frost-free period under conditions of the traditional agricultural practice E and the soil mulching Em . The points in the graph lie on the line Vb(i) = Ve(i-1) + Vw , (26) where Vb(i) is initial (spring) soil water storage of the current year, Ve(i-1) is final (autumn) soil water storage of the previous year, Vw is spring soil water change in the case of traditional agricultural practice. To determine the initial soil water storage, which start the calculations, the condition Vb V fc should be valid, where V fc is the field capacity. The basic parameters of the evapotranspiration estimation are universal, i.e., they can be used for calculations in different physic-geographical zones. With this scheme one can estimate the total soil water storage that include both the productive component of soil water storage (available water for plants) and unavailable water for plants (immobile water). The modifica- starting from the axis cross­section at an angle of 45O and below this line. It indicates the fact, that evapotranspiration for a frost-free period in case of the soil mulching either retains its initial value or decreases compared to evapotranspiration under conditions of a traditional agricultural practice. Mulch water Fig. 5. Spring wheat field annual evapotranspiration probability curves of a frost-free period in the case of the traditional agricultural practice E (circles) and soil mulching Em (full circles). evaporation calculated from the actual rain event does not reach 4 mm. An exception are eight months out of 197 considered (in 4% of cases), when evaporation exceeds 4 mm for each rain. This implies that in the average annual conditions, only the precipitation retained in the mulch is evaporated, i.e., the first stage of evaporation (described by (20)) takes place. Fig. 3 shows a spatial distribution of average annual values of the relations Em / E . The equality Em / E = 1 is characteristic of the arid part of the area under study, while the wet regions are characterized by Em / E < 1 . On average, in case of the soil mulching this ratio in the forest-steppe and steppe zones can be 0.94, i.e., the evapotranspiration in a case of the soil mulching can decrease by 6%. In the average, the evapotranspiration at certain stations can decrease for a frost-free period to 23­25% (Vladimir-Volynskiy and Krasnodar stations), which in absolute values amounts to 105 and 135 mm, respectively. Fig. 4 can give an idea for individual years between the evapotranspiration with the traditional agricultural practice and in case of the soil mulching. In individual years the mulching leads to the decreased evapotranspiration and then the water accumulated in the soil can lead to its increase (as a result of the transpiration increase). It is mainly characteristic of the Bezenchuk and Ershov stations in the arid zone. At the Kamennaya Step and Mironovka stations, in most cases, the evapotranspiration noticeably decreases in the presence of the mulch. It is illustrated by the graph in Fig. 2b). The maximum decrease of the evapotranspiration over a number of the years studied ranged in case of mulching from 53 mm (Bezenchuk station) to 124 mm (Mironovka station). The probability curves shown in Fig. 5 also give an idea of inter-annual variability of the evapotranspiration when traditional agricultural practice and soil mulching is used. The analysis of results demonstrated (Fig. 5 and Table 1) mean evapotranspiration in case of the traditional agricultural practice E and soil mulching Em are almost the same for the Bezenchuk and Ershov stations. The difference between them is within the accuracy of the calculation method. The evapotranspiration of the mulched soil of the Gigant and Odessa stations decreases on average by 3-4% (8­11 mm). The maximum evapotranspiration decrease by the soil mulching can be expected in Kamennaya Step (by 7% or 25 mm) and in Mironovka (by 14% or 82 mm). Mean standard deviations of the evapotranspiration for mulched soil decrease in the range from 14 (Mironovka) to 51 mm (Ershov). Under conditions of a traditional agricultural practice the range of the mean standard deviation changes between 56­78 mm. The coefficient of variation of the evapotranspiration when the soil is mulched is also significantly lower than in the conditions of a traditional agricultural practice: from 0.04 in Mironovka to 0.17 in Ershov. CONCLUSION The results of calculations and their analysis for the arid regions of territory under study indicate the evapotranspiration of the mulched soil does not change significantly ( Em = E ), but the relation between the transpiration and soil evaporation (evapotranspiration structure) changes. In the wet regions, along with the change of evapotranspiration structure, the evapotranspiration of mulched soil decreases ( Em < E ), which can result in a change of the hydrological cycle natural structure. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Hydrology and Hydromechanics de Gruyter

Evapotranspiration changes in the forest-steppe and steppe zones under soil mulching

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de Gruyter
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Copyright © 2013 by the
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0042-790X
DOI
10.2478/johh-2013-0019
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Abstract

This paper contains the method and results of calculation evapotranspiration and its structure ­ transpiration and evaporation from traditionally tilled and mulched soil. The data considered indicate that in the arid regions of territory under study the evapotranspiration when soil is mulched does not change significantly and only the relation between the transpiration and soil evaporation changes. In the wet regions, evapotranspiration decreases when the soil is mulched which can result in a certain disturbance of the natural structure of the hydrological cycle and possible overmoistening soils. Keywords: Evapotranspiration; Evapotranspiration structure; Traditional agricultural practice; Soil mulching. INTRODUCTION The abundance of light and heat together with high soil productivity promoted the transformation of the southern Russian regions into an agricultural landscape. But these regions are characterized by insufficient precipitations (Shumova, 2001), which led to a wide-spread irrigation. The economic activities resulted in the local land overmoistening and salinization leading to the degradation of high-productive chernozem soils and removal of land from the agricultural use. Recently, in the agricultural production, the interest is paid to the nature friendly agricultural practice associated with the minimum man impact on the environment (Agricultural ecosystems, 1987). Natural ecosystems have on the soil surface a layer of felt and a litter of died-of plant mass, which allows them to optimally use the biosphere resources. Using the technology of a direct seeding and minimal soil treatment (Allen, 1985) with soil mulching as one of its links, the agroecosystem can be brought to a more natural form. The soil mulching promotes a substantial decrease in soil evaporation and an increase in transpiration, which results in the enhanced water supply of agricultural crops (Shumova, 1997). If under conditions of traditional agricultural practice in the forest-steppe and steppe zones part of transpiration to evapotranspiration in the spring wheat field amounts to 30­40% (Shumova, 2000) then in case of soil mulching with the straw it can amount to more than 60% (Shumova, 1997). However, a question is whether the magnitude of the evapotranspiration under soil mulching is changing or the structure of evapotranspiration changes. Presently, the problem is vital, as the reduced evapotranspiration in case of the soil mulching can result in the changed structure of a hydrological cycle: in the water accumulation in the soil, which is undesirable under conditions of the modern hydromorphism (Novikova and Nazarenko, 2007). Theoretical basis of the calculation method used in this paper is presented in papers of Budagovskyi and Novak (2011a,b). The objective of this work is to estimate the influence of soil mulching on the spring wheat field evapotranspiration in the foreststeppe and steppe zones. The problem is based on the numerical experiment, which to the evapotranspiration calculation under conditions of a traditional agricultural practice and soil mulching. The approach used in this work allows accounting for quantitative demands to the evapotranspiration of agricultural crops during their development. METHOD The evapotranspiration E in a general case includes the soil evaporation Es and the transpiration Et (Budagovskiy, 1964; Shumova, 2003) E = Es + Et . (1) The empirical formulas (Budagovskiy, Shumova, 1976) are used for calculating soil evaporation Es = Es0 Ve + 1- e ), (2) (3) (4) Es0 = b1s Ds d + b2 Re-s - B , s = e-1.1 , Ds = 0.8u u 1/2 + 0.7 (5) b1 = 0.7 , 1+ 1.56 b2 = 17.1T 0.026 , 1+ 1.56 (6) ( 235 + T ) e 235+T , 2 (7) where Es0 is potential soil evaporation (mm day-1), is empirical parameter dependent on hydrophysical soil properties (mm-1), V is available soil water storage of soil root layer (mm), P is precipitation (mm day-1), b1 and b2 are function of air temperature, s is function of the leaf area index, Ds is function of wind speed, d is air humidity deficit at 2 m (mb), R is net radiation (cal cm-2 day-1), s is coefficient dependent on the geographical latitude and season of year (Budagovskiy, 1964), is leaf area index (cm2 cm-2), B is heat flux in the soil (cal cm-2 day-1), u is wind speed at 2 m (m s-1), is the slope of the saturated water vapor pressure as a function of air temperature (mb OC-1), T is air temperature at 2 m (OC). Transpiration is calculated as follows E t0 = V E t0 Vcr k = 1- e )E s0 -P, (17) at at V Vcr V <Vcr , (8) m= 1 b D d + b2 R 1- 2e-s - Vcr 1 1 - P/ Es0 - 1 - 2 1- e Es0 + b2 B + Es0e . )( (18) Et0 = b1D 1d + b2 R 1- 2e-s - 1- 2 B - 1- 2 Es , (9) D = c1u c2u1/2 + 1 u = u + 0.4 , (10) 1 = 1- e- 2 = e-0.25 , (11) where Et0 is potential transpiration (mm day-1), Vcr is critical The calculation is performed with a ten day time step and covers a frost-free period (a period between a complete snow melting in spring and appearance of air temperature below zero in autumn). First, soil water storage then, soil evaporation and transpiration are calculated. A detailed assessment of the accuracy of the calculation scheme described is given in Shumova (2003). In case of the soil mulching, the scheme of the evapotranspiration calculation given above is used too. The differences are only in the use of other relations describing the soil evaporation. The estimation of the soil evaporation in case of mulching is based on the formula describing evaporation through a driedup soil layer (Budagovskyi, 1964), which for the mulching conditions is written (Budagovskiy and Grigorieva, 1991): available soil water storage (mm), D and u are function of wind speed, 1 and 2 are functions of the leaf area index, Es(m) = c1 = 2.4 , c2 = 4 m 1/2 -1/2 are coefficients determined from the Es0 , 1+ z m (19) field experiments. Critical available soil water storage in a one meter soil layer are described with the relation where Es(m) is soil evaporation under soil mulching, is parameter depending on diffusivity of water vapour in the mulch layer and on its heat conductivity, z m is thickness of the mulch layer. It should be taken into account that after each rainfall the mulch itself also retains some precipitation amount. On that basis two stages of the soil evaporation are considered in the presence of the mulch. The first stage is evaporation resulted Vcr = 60 + 4.2E0 , (12) (13) E0 = b1D 1d + b2 ( R - B ) , where E0 is potential evapotranspiration (mm day-1). Available soil water storage is determined according to the formula (Budagovskiy and Shumova, 1976) Ve = Vb + k / m e-m - k / m , (14) where Ve and Vb are available soil water storage (mm), at the beginning and at the end of the calculation time interval, respectively, is time interval (day). If the available soil water storage is above or equal to its critical values V Vcr , calculated in accordance with (12) and (13), k and m are calculated with the relations k = b1D 1d + b2 R 1- 2e-s - 1- 2 B + + 2 1- e )E s0 - P, (15) m = 2 Es0e (16) In the case of the available soil water storage is below its critical value V < Vcr , k and m are calculated with the formulas Fig. 1. Location of agrometeorological stations (AS). Dotted lines denote the boundaries of the forest-steppe and steppe zones (Berg, 1947; Berg, 1952). Circles denote agrometeorological stations with average annual data used for calculations; Full circles denote the AS annual data; 1 ­ Kazan, 2 ­ Mikhailov, 3 ­ Samara, 4 ­ Bezenchuk, 5 ­ Orenburg, 6 ­ Glukhov, 7 ­ Saratov, 8 ­ Nizhnedevitsk, 9 ­ Ershov, 10 ­ Uralsk, 11 ­ Kamennaya Step, 12 ­ VladimirVolynskiy, 13 ­ Mironovka, 14 ­ Poltava, 15 ­ Dzhanybek, 16 ­ Belovodsk, 17 ­ Novaya Ushitsa, 18 ­ Kirovograd, 19 ­ Konstantinovskiy, 20 ­ Kharabali, 21 ­ Mariupol, 22 ­ Kishinev, 23 ­ Kherson, 24 ­ Gigant, 25 ­ Odessa, 26 ­ Sarata, 27 ­ Krasnodar, 28 ­ Zolotushka. N. Shumova Fig. 2. Average annual values (a) and annual values (b) of the spring wheat field evapotranspiration in the case of the traditional agricultural practice E and from mulched soil Em . from the mulch drying, and the second stage is soil evaporation through the mulch layer. The first stage, the evaporation of the precipitation retained by the mulch is described with the formula (Budagovskiy and Grigorieva, 1991) Es(m) dt = E t0(m) E =V t(m) E t0(m) V cr at at V Vcr V 0.1, 0.5, 1.0, 5.0, 10.0, 20.0, and 30.0 mm. In this case, the result of calculations are monthly values of the soil evaporation; the ten day values are determined from the monthly values proportionally to the ten day values Es0 . When using the straw mulch (as the most attractive from the ecological viewpoint), according to the numerical experiments (Dzhogan nad Gusev, 2003; Gusev and Dzhogan, 2000), the thickness of its layer noticeably influences the soil evaporation only up to the thickness 4­5 cm. Thus, a 4­5 cm straw mulch layer (which corresponds to a mass of 10 t ha-1) is optimal that practically results in the maximally possible reduction in the soil evaporation. The technique of calculations of the evapotranspiration and soil water storage in the presence of the mulch is the following. The soil evaporation Es(m) , as was shown above, is determined independently from the precipitation and potential soil evaporation Es0 , which is determined using Eq. (3). The transpiration where m is parameter that depends on the mulch ability to Et0(m) = b1D 1d + b2 R 1- 2e-s - - 1- 2 Es(m) . - 1- 2 B (22) The available soil water storage under conditions of the sufficient water content V Vcr can be determined from the water balance relation Ve = Vb + (P - Es(m) - Et0(m) ) . (23) Fig. 3. Average annual values Em / E . Full circles denote the cases when Em = E . Et(m) is determined from the relation similar to (8), which in case of mulching becomes as follows When available soil water storage is below critical values V < Vcr , available soil water storage is determined by formula (14) at Fig. 4. Spring wheat field annual evapotranspiration for a frost-free period in the case of the traditional agricultural practice E (circles) and soil mulching Em (full circles). Table 1. Interannual variability of the spring wheat field evapotranspiration under a traditional agricultural practice and soil mulching. Station number 4 9 11 13 24 25 cients. Station Bezenchuk Ershov Kamennaya Step Mironovka Gigant Odessa Traditional agricultural practice E (mm) CV ( E) E (mm) 340 305 388 456 404 387 Soil mulching E (mm) E m (mm) m 334 308 363 374 396 376 37 51 39 14 26 29 CV ( E m) Note: E and E m are average values, E and E are standard deviations, CV ( E) and CV ( E ) are variation coeffim k = Es(m) - P , m = Et0(m) / Vcr . (24) (25) Evapotranspiration, as in case of the traditional agricultural practice, is calculated with a ten day time step and covers a frost-free period. The evapotranspiration calculations are based on standard observations of agrometeorological and actinometrical stations. The leaf area index is determined according to Shumova, (2003). Under conditions of the traditional agricultural practice (in the absence of mulching layer), the soil water storage values measured at the agrometeorological stations are taken in all cases as initial conditions. In the presence of the mulch the selection of the initial soil water storage that start the calculation has the following characteristic features. Selection of the initial soil water storage Vb , in the first ten day after complete melting of the snow cover in spring, should correspond to the same value as under conditions of the tradi- N. Shumova tional agricultural practice. When performing the calculations for several years, in the first year, the first measured soil water storage is taken as the initial water storage Vb . To determine the initial soil water storage in the next year, the spring soil water content, observed under traditional agricultural practice, is added to the final soil water storage Ve , which are observed in the last ten days with air temperature above zero in the previous autumn tion of the above scheme can be used for detail study of the water balance formation in the root layer in case of mulching (Gusev and Dzhogan, 2000); the scheme provides a detail description of physical processes in mulch layer and in the soil under the mulch in the frost-free period and for estimating the influence of a straw mulch on the soil water replenishment in spring to use a complex of models of the soil water formation in the winter-springtime as suggested in Gusev (1993). RESULTS AND DISCUSSION The average annual values of the spring wheat field evapotranspiration were calculated by the above relations, using the data from 28 agrometeorological stations (Fig. 1); for six of these stations (Bezenchuk, Ershov, Kamennaya Step, Mironovka, Gigant, and Odessa) the evapotranspiration was calculated for individual years. The calculations are performed in two variants: in the absence of the mulching cover and in the presence of a 5-cm layer of the straw mulch. Fig. 2a) shows relations between average annual values of the spring wheat field evapotranspiration for a frost-free period under conditions of the traditional agricultural practice E and the soil mulching Em . The points in the graph lie on the line Vb(i) = Ve(i-1) + Vw , (26) where Vb(i) is initial (spring) soil water storage of the current year, Ve(i-1) is final (autumn) soil water storage of the previous year, Vw is spring soil water change in the case of traditional agricultural practice. To determine the initial soil water storage, which start the calculations, the condition Vb V fc should be valid, where V fc is the field capacity. The basic parameters of the evapotranspiration estimation are universal, i.e., they can be used for calculations in different physic-geographical zones. With this scheme one can estimate the total soil water storage that include both the productive component of soil water storage (available water for plants) and unavailable water for plants (immobile water). The modifica- starting from the axis cross­section at an angle of 45O and below this line. It indicates the fact, that evapotranspiration for a frost-free period in case of the soil mulching either retains its initial value or decreases compared to evapotranspiration under conditions of a traditional agricultural practice. Mulch water Fig. 5. Spring wheat field annual evapotranspiration probability curves of a frost-free period in the case of the traditional agricultural practice E (circles) and soil mulching Em (full circles). evaporation calculated from the actual rain event does not reach 4 mm. An exception are eight months out of 197 considered (in 4% of cases), when evaporation exceeds 4 mm for each rain. This implies that in the average annual conditions, only the precipitation retained in the mulch is evaporated, i.e., the first stage of evaporation (described by (20)) takes place. Fig. 3 shows a spatial distribution of average annual values of the relations Em / E . The equality Em / E = 1 is characteristic of the arid part of the area under study, while the wet regions are characterized by Em / E < 1 . On average, in case of the soil mulching this ratio in the forest-steppe and steppe zones can be 0.94, i.e., the evapotranspiration in a case of the soil mulching can decrease by 6%. In the average, the evapotranspiration at certain stations can decrease for a frost-free period to 23­25% (Vladimir-Volynskiy and Krasnodar stations), which in absolute values amounts to 105 and 135 mm, respectively. Fig. 4 can give an idea for individual years between the evapotranspiration with the traditional agricultural practice and in case of the soil mulching. In individual years the mulching leads to the decreased evapotranspiration and then the water accumulated in the soil can lead to its increase (as a result of the transpiration increase). It is mainly characteristic of the Bezenchuk and Ershov stations in the arid zone. At the Kamennaya Step and Mironovka stations, in most cases, the evapotranspiration noticeably decreases in the presence of the mulch. It is illustrated by the graph in Fig. 2b). The maximum decrease of the evapotranspiration over a number of the years studied ranged in case of mulching from 53 mm (Bezenchuk station) to 124 mm (Mironovka station). The probability curves shown in Fig. 5 also give an idea of inter-annual variability of the evapotranspiration when traditional agricultural practice and soil mulching is used. The analysis of results demonstrated (Fig. 5 and Table 1) mean evapotranspiration in case of the traditional agricultural practice E and soil mulching Em are almost the same for the Bezenchuk and Ershov stations. The difference between them is within the accuracy of the calculation method. The evapotranspiration of the mulched soil of the Gigant and Odessa stations decreases on average by 3-4% (8­11 mm). The maximum evapotranspiration decrease by the soil mulching can be expected in Kamennaya Step (by 7% or 25 mm) and in Mironovka (by 14% or 82 mm). Mean standard deviations of the evapotranspiration for mulched soil decrease in the range from 14 (Mironovka) to 51 mm (Ershov). Under conditions of a traditional agricultural practice the range of the mean standard deviation changes between 56­78 mm. The coefficient of variation of the evapotranspiration when the soil is mulched is also significantly lower than in the conditions of a traditional agricultural practice: from 0.04 in Mironovka to 0.17 in Ershov. CONCLUSION The results of calculations and their analysis for the arid regions of territory under study indicate the evapotranspiration of the mulched soil does not change significantly ( Em = E ), but the relation between the transpiration and soil evaporation (evapotranspiration structure) changes. In the wet regions, along with the change of evapotranspiration structure, the evapotranspiration of mulched soil decreases ( Em < E ), which can result in a change of the hydrological cycle natural structure.

Journal

Journal of Hydrology and Hydromechanics – de Gruyter

Published: Jun 1, 2013

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