Modeling the E ects of Crust on Rain In ltration in Vegetated Sand Dunes in Arid Desert

Transcription

1 Arid Land Research and Management, 15:41È48, 2001 Copyright # 2001 Taylor & Francis /01 $ Modeling the E ects of Crust on Rain In ltration in Vegetated Sand Dunes in Arid Desert LI TAO XIAO HONGLANG LI XINRONG Shapotou Desert Research Station Cold and Arid Regions Environmental and Engineering Research Institute Chinese Academy of Sciences Lanzhou, Gansu, China Shapotou Desert Research Station is located at the southeast edge of T engger Desert in China. Shapotou sand dunes have been vegetated to stabilize them. T he vegetated dunes have been completely covered by a natural crust. Rain inðltration was reduced by 36% to 74% on the crusted site while there was no reduction of rain inðltration on the sandy site. T he inðltration rate was positively correlated with total rainfall and negatively correlated with rain intensity. Runo on the crusted site provided water for vegetation growth in the hollows between sand dunes. T he crust signiðcantly impeded increased soil moisture. T o increase soil moisture in the surface 40 cm layer, rainfall had to be above 8.5 mm on the slopes of sand dunes on the crust site, and increased with rain intensity. In the Shapotou area, the amount of rainfall that vegetation on slopes of sand dunes could utilize, was reduced to about 40 mm per year by the crust, although the mean annual precipitation was approximately 180 mm. It is expected that vegetation cover will continuously degrade in the future. T o maintain vegetative cover adequate to stabilize the active sand dune, it was necessary to break part of the crust at slope of sand dune. Keywords soil moisture, precipitation, stabilization of sand dune, Shapotou Shapotou Desert Research Station is well known for research on sand storms, desertiðcation control, and arid ecological systems. It was created in 1956 to look for e ective methods to protect the worldïs Ðrst desert railway from migrating sand dunes. Current research issues include plant ecology and physiology, arid land management, desertiðcation control, rehabilitation, and agriculture. To protect the desert railway, sand dunes bordering the railway have been vegetated without irrigation since A surface crust, 0.8 to 5.9 cm thick, has formed in the vegetated area during the past 43 years (Figure 1). The crust seems to hinder the inðltration of rainfall, and the soil moisture in subsoil tends to decrease year by year (Figure 1). This has resulted in the loss of some shrubs, and vegetation cover has started to decline. The plant community on Received 17 March 2000; accepted 29 June This study was funded by the Programs of Chinese Academy of Sciences (KZ95T-04-01, KZ95-S1-218), and was part of a project of National Sciences and Technologies Committee ( ). It was also supported by the Fund of Chinese Academy of Sciences for students returned from foreign countries. We are grateful to our supporters. Address correspondence to Li Tao, Cold and Arid Regions Environmental and Engineering Research Institute (CAREERI), Chinese Academy of Sciences; West Donggang Road, 260, Postal Code: ; Lanzhou, Gansu, China. litaoguo@public.lz.gs.cn 41

2 42 L i T ao et al. FIGURE 1 The evolvement of vegetative covers, soil moisture, and thickness of crust since sand dunes. (All Ðgures showed in this graph were investigated in August Vegetative covers were taken from di erent 100 m 2 sampling areas on the upper part of sand dunes which were vegetated in di erent years in the Shapotou Desert Experimental Research Station. Thickness of crust and soil moisture were mean of 6 to 9 points that were randomly selected in the 100 m 2 sampling area.) the top portion of dunes introduced in 1956, which consisted of three species of shrubs (Hedysarum scoparium Fisch. & Mey., Artemisia ordosica Krasch., Caragana korshinskii Kom.), has gradually degraded and evolved to a native community composed of small shrubs (Artemisia ordosica, Caragana korshinskii) and annual grasses (Eragrostis poaeoides Beauv., Bassia dasyphylla (Fisch. & Mey.) O. Kuntze, Salsola ruthenica Iljin, Corispermum hyssopifolium L., Agriophyllum arenarium M.B.) (Shi Qinghui and Liu Jiaqiong 1995). Plant cover has decreased from near 30% to about 10%. The lower portion of the dunes, including the spaces between dunes, is occupied by various shrubs and annual grass with 30% to 40% cover. Lichens almost cover the soil surface, and a large number of agrobacteria is present in the area (Zhang et al. 1993). It is important to understand how the crust a ects inðltration, soil moisture in the subsoil, and growth of vegetation in order to determine whether a Ðeld experiment to break the crust should be conducted. How to break the crust would be studied in a future Ðeld experiment so that inðltration of precipitation and soil moisture could be efficiently improved, and runo and water erosion, and the risk to activate Ðxed sand dunes could be reduced to as low a level as possible. Consequently, plant cover could be maintained at adequate density, so that active sand dunes could be permanently stabilized. The purpose of this study was to understand how the crust a ects the inðltration of rainfall and soil moisture. Materials and Methods Shapotou area is located at the southeastern edge of the Tengger Desert (longitude E , latitude N37 279, elevation 1500 m). In this area, the annual average air

3 E ects of Crust on Rain InÐltration 43 temperature is 7.4 C, with the highest and lowest air temperatures are 43 C and ] 27.2 C, respectively. The annual precipitation is 180 mm with 60%È70% occurring from June to September. Sand and dust storms occur about 174 days per year. It was difficult to measure the impact of precipitation by Ðeld observations because of inadequate measuring methods. Therefore, the Limburg Soil Erosion Model (LISEM) (De Roo and Wesseling 1995; De Roo, Wesseling, and Ritsema 1996; Li Tao 1998), a physical water erosion model, was used to simulate the e ects on the crust of precipitation. To use LISEM, the saturated hydraulic conductivity, saturated water content, and water potential characteristics of the crust and sand must be obtained. The sand dune area, vegetated in 1956 and completely covered by a crust, was the main study area. Because the dunes in the vegetated area had not migrated during the last 43 years, dust particles had been deposited on the surface of vegetated sand dunes. The dust particles were then conglomerated into the current crust formed comprehensively by occasional rain and some activity of soil microorganisms. The amount of dust brought by wind from other places was about 1.6 mm per year. The percentage in the crust of soil particles whose diameter was less than 0.05 mm was more than 50.6%, and the maximum diameter of soil particles was 0.25 mm (Xiao Honglang, Zhang, and Li 1997). Ninety-nine percent of particles of the dune were between 0.05 and 0.25 mm in diameter (Table 1). The study site of m 2 was located in a typical microtopographical situation (Figure 2). Precipitation e ects on rain were simulated by LISEM on two di erent surface conditions at this site. The Ðrst condition, called a crust site, had sand dunes covered with a crust 10 to 28 mm thick. The second, called a sandy site, exhibited a sandy soil proðle throughout the dunes. The constant head method introduced by Stolte (1997) was used to determine the saturated hydraulic conductivity of both crust and sand samples. To measure soil water potential, the method of the pressure plate extractor (15Bar Ceramic Plate Extractor, made by Soil Moisture Equipment Corp., U.S.A.) introduced by Stolte and Veerman (1997) was used. The method introduced by Weng Deheng (1979) was used to measure the saturated water content. Leaf area index, vegetation cover, and height of introduced and natural vegetation needed in the LISEM were measured in Ðeld. The study area was divided into seven sampling areas at the top of the dunes on the LS and WS, the middle of the dunes on the LS and WS, the bottom of the dunes on the LS and WS, and the hollow area between the dunes (Figure 2). Seven to ten 1 m 2 sample points were randomly chosen for measuring plant data. One typical soil proðle in a sampling area was randomly chosen to take three samples, such as crust, TABLE 1 The physical and chemical characteristics of crust and sand soil in studied area a Saturated Saturated Organic Number of soil soil water hydraulic matter microorganisms content conductivity Soil type (g kg { 1 soil) (10 6 kg { 1 soil) (cm 3 cm { 3 ) (cm day { 1 ) Crust Sand soil under crust Deep sand soil a Figures in this table were average of all samples in the studied area. Thickness of crust varied from 1 cm to 2.8 cm at di erent points. The sand soil under crust was 6 cm. The depth of deep sand soil was between 10 cm and 60 cm.

4 44 L i T ao et al. FIGURE 2 The microtopographical site and points for study. TABLE 2 Ten-year precipitation data used to estimate rainfall intensity and duration a Rain Raining Total amount Estimated Code of intensity duration of rain time rain event (mm h { 1 ) (min) (mm) (min) a Estimated time must exceed the raining duration so that all runo could reach the outlet point of the study area. The estimated time is the running time of LISEM.

5 E ects of Crust on Rain InÐltration 45 subsoil, and sandy soil, for measuring soil hydraulic data. Soil hydraulic data were transformed into a single table and used by LISEM (Table 1). In a same sampling area, plant leaf index, and cover and height of vegetation data were used to indicate the value of the sampling area. Variances of the sample means were less than 5%. The data were used to create a standard map with the same grid size and area. The maps were utilized as input data by LISEM. Rainfall Data Rain intensity data was unavailable. Probable rain intensity and amount could be estimated from precipitation records of the last 10 years (Table 2). Estimated rain events 7 and 15 occurred only once in Shapotou in the past 10 years. Results and Discussion In the vegetated area, the crust retarded rainfall inðltration, and induced runo and erosion. However, the crust was important to prevent aeolian sand movement and migration of the sand dune. The intensity and total amount of precipitation determined the e ects of the crust on inðltration and soil moisture. Inültration On the crust site, the rate and amount of inðltration could be described by the following equations according to the data simulated by LISEM : IR ] 0.40RI RT [R R c (P )] (1) IA 5 ] 0.19 ] 0.04RI RT (R ) (2) where IR 5 inðltration rate as a percentage [inðltration rate 5 (total amount of inðltration)/(total precipitation) 3 100], RI 5 rain intensity in mm h { 1, IA 5 total amount of inðltration in mm, RT 5 total precipitation in mm, R 5 relationship value, and R 5 critical relationship value for 95% reliability. The inðltration rate was negatively related to rain intensity and positively related to total precipitation. Under estimated rain intensity and total precipitation, the inðltration rate could not exceed 64% of total precipitation or be higher than 26% of total precipitation. In addition, the total amount of inðltration was less than 20 mm. For example, when rain intensity was 1 mm h { 1 and total precipitation was 32 mm, the precipitation event should continue for 32 hours with the inðltration rate at 63.9% of total rain and the total amount of inðltration at mm. In Shapotou Station, the total precipitation of a continuous precipitation event never exceeded 30 mm. Therefore, the possible maximum inðltration rate was 63%, and total amount of inðltration was not more than 18 mm during the event. Runo, which was about 36% to 74% of total rainfall, Ñowed to and was stored in the hollow between the dunes (Figure 2). Moreover, the thicker the crust, the more water was captured by the crust during a rain episode. Therefore, the amount and duration of soil water was greater in the hollows than on the slopes of the sand dunes. This partially explains why vegetation cover in the hollows was greater than other areas. In addition, the large amount of runo induced erosion. Maximum eroded depth was about 1.5 mm per rainstorm, and occurred mainly at the top and middle points of slopes. The runo carried crust particles to the hollows where they were deposited. This accounted for the much thicker crust on the hollow surface than that on the slopes of sand dunes. On the sandy site, the inðltration rate was always 100%. No runo was produced, although rain intensity was considered to be 64 mm h { 1 (rainstorm). Comparing the sandy site with the crust site, the crust reduced the amount of rain

6 46 L i T ao et al. FIGURE 3 The rain amount and rain intensity if the soil moisture of surface 40 cm soil layer would be increased. (The data indicated in the Ðgure simulate results by LISEM, and the reliability is larger than or equal to 95%.) inðltrated into the subsoil by 36% to 74%. The crust was a barrier for inðltration of precipitation. Therefore, the crust negatively e ected on soil moisture and growth of vegetation on the slope of the sand dunes. The Improvement of Soil M oisture As discussed above, soil moisture in the hollows could be easily increased because large amounts of runo accumulated there. Vegetation cover was high because of high soil moisture content. Precipitation was the only water source to improve soil moisture in the desert area. During a rainfall event, precipitation inðltrates into the soil, and soil moisture is increased gradually. The depth to which soil moisture increased after a rain event depended on rain intensity, the total amount of rain, and soil type. When soil moisture at surface 40 cm layer increased after a rain event, the event beneðted vegetation (Qiu Guoyu and Shi Qinghui 1993) in the sand dune area. In this case the rain event was considered to be an e ective rain. An e ective rain

7 E ects of Crust on Rain InÐltration 47 should consist of appropriate rain intensity and amount according to the modeling data (Figure 3). On the crust site, for soil moisture in the surface 40 cm layer to be improved, the total amount of rain had to be more than 8.5 mm. As Figure 3 shows, if rain intensity is increased, the total amount of rain had to increase to enhance the soil moisture in the surface 40 cm layer. If rain intensity was the same, a higher amount of total rain was needed at the top points than at the middle points of slopes. Because the gradient of the leeward slopes was higher than that of the windward slopes, a higher total amount of rain was needed to increase soil moisture in the surface 40 cm layer on leeward slopes than on windward slopes. On any point of the sandy site, the total amount of rain needed to increase soil moisture in the surface 40 cm layer exponentially decreased as rain intensity increased (Figure 3). The velocity of the water moving down to deeper soil layers increased as rain intensity increased. Soil moisture could be increased for almost all rain events. Generally, the presence of crust reduced increases in soil moisture after a rainfall event. In the desert area, most rain events did not reach the necessary conditions for amount and intensity. Therefore, crust was harmful for vegetation in the vegetated sand dune area. According to the meteorological data of Shapotou Desert Research Station, the number of rain events more than or equal to 8.5 mm is only about eight per year, and the annual total amount is about 107 mm. The possible amount of water inðltrated into the subsoil is less than 40 mm per year. According to the water consumption of plants given by Zeng Wenbing, Jie Hongmet, and Wei Baowen (1993) and Feng Jinchao and Chen Hesheng (1993), the water that inðltrated into the subsoil met only 50% of that needed by shrubs (Caragana korshinskii and Artemisia ordaosica) presented at top and middle points of sand dune slopes of the studied site. This may mean that vegetation cover would be reduced by 50% in the future if the amount of rain inðltrating into the subsoil remains at 40 mm per year. Therefore, it is necessary to break part of crust at the top and middle points so that more precipitation could inðltrate into the soil to maintain high vegetation cover. The composition of the vegetation tends to degrade because the crust obstructs the inðltration of precipitation. A Ðeld experiment should be conducted as early as possible so that the risk of wind erosion to broken crust could be evaluated; the improvement of soil moisture after breaking the crust could also be studied in detail. Conclusions 1. In the vegetated area of Shapotou Sand Dunes, soil crust reduced the inðltration of rain by 36% and 74% compared to a sandy site. It exerted negative e ects on soil moisture and growth of vegetation. 2. On the crust site, for soil moisture in the surface 40 cm layer to be increased, the total amount of rain had to be above 8.5 mm per event. In addition, if the rain intensity increases, the total amount of rain should also increase. 3. In the vegetated area of the crust site, at the top and middle points on sand dune slopes, most rain events could not beneðt the vegetation because they could not match the required conditions for total amount and intensity. As a result, vegetative cover will degrade in the future. For improved vegetative growth and cover, it is necessary to break a part of the crust at the top and middle points at the sand dune slopes. References De Roo, A. P. J., and C. G. Wesseling L ISEM: L imburg Soil Erosion Model, A user guide. Department of Physical Geography, Utretch University, The Netherlands.

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