Tan Lu, Wang Zhong-bo*, Huang Jin-bai, and Yasuda Hiroshi

1 College of Water Conservancy and Civil Engineering, Northeast Agricultural University, Harbin 150030, China

2 Arid Land Research Center, Tottori University, Tottori 680-0001, Japan

Introduction

Chinese Loess Plateau is a vast semiarid and high risk region of desertif i cation (Wang and Takahashi, 1999),and Loess Plateau has been described as one of the most serious soil erosion areas in the world (UNEP,1997; Sun, 1998). Re-vegetation is recognized as an important means to control soil erosion and protect land from desertification (Bu et al., 2002; Jia et al.,2010), fast growing tree and shrub species have been commonly used for vegetation restoration in the Loess Plateau (Wu and Yang, 1998; Huang et al., 2003). A great effort has been made to plant trees and grasses on slope land since the end of 1950s. Thereby, about 24% erosion area has been controlled, and vegetation coverage has been increased from 6.5% in 1970s to 11% in 1995 in the Loess Plateau (He et al., 2003,Chen et al., 2008a). However, large-scale vegetation restoration has also aggravated water scarcity (Li,2001; Shangguan and Zheng, 2006). Water is a key factor for vegetation restoration (Huo et al., 2008). In most regions of the Loess Plateau, the average annual rainfall ranges from 300 mm in the northwest to 650 mm in the southeast; however, the relevant mean annual evaporation varies from 623.8 to 1 254.0 mm.Scarce and uneven distribution of rainfall in addition to intense evapotranspiration causes the def i ciency of annual water resources (Zheng et al., 2005; Chen et al.,2008b). Def i ciency of water resources mainly hampers vegetation restoration in the Loess Plateau (Yang,1996; Hinokidani et al., 2010). The most serious erosion area is the wind-water erosion crisscross region in the northern Loess Plateau, tendency of desertif i cation in this region has become more severe than that in the southern region of the Loess Plateau due to the long-term water and wind crisscross erosion complimented by low precipitation (Cheng et al.,2007; Huang et al., 2008). The studies on the effects of soil moisture conditions on vegetation growth at the wind-water erosion crisscross region have received more attention (Fan et al., 2006). Determination of water consumption of evapotranspiration (ET)over typical vegetation is significant for construction and management of artificial vegetation in win-water erosion crisscross region (Wang et al., 2009). Kimura et al. (2005a)pointed that there was a potential to increase grassland vegetation according to drought index of the wind-water erosion crisscross region.In the current study, the research activities were conducted in the Liudaogou Catchment with aim to estimate ET over the grassland and assess seasonal moisture availability. Moreover, to provide the basis for studies on dynamic functional analysis of soil moisture and relationship between soil water and crop growth at wind-water erosion crisscross region in the northern Loess Plateau.

General Situation of Study Location

The Liudaogou Catchment (area: 6.89 km2, 110˚21′-110˚23′ E longitude and 38˚46′-38˚51′ N latitude)was chosen as the study location, because it represents diverse landscape types in terms of geology, morphology, soil, hydrology and climatic conditions of the wind-water erosion crisscross region in the northern Loess Plateau environment (Zhu and Shao,2008; Huang et al., 2008). This catchment is situated at an elevation ranging from 1 094.0 m to 1 273.9 m above sea level. Annual precipitation is only 430 mm with uneven monthly distribution, while more than 70% of the total is received in rainy season from June to September. Terrain is considerably complicated,many gullies in various scales spread around the main river channel. The rigorous wind and water crisscross erosion cause severe degradation of the ecological environment and increase the risk of desertification.The natural vegetation has been severely destroyed,and there is almost no large-area distribution, and the vegetation coverage is less than 25% (Zheng et al.,2005; Han et al., 2009; Huang, 2010).

Materials and Methods

Data acquisition

The main vegetation was grassland in the Liudaogou Catchment. The grasses were mainly Chinese wildrye(Aneurolepidium chinense), lyme grass (Leymus arenarius), Halymenia dentate (Halymenia dilatata),timothy grass (Phleum pratense)and a small amount of alfalfa (Medicago sativa). Observation of the soil moisture was carried out at the upstream of the Liudaogou Catchment (Soil moisture sensor: model number: S-SMC-M005, HOBO Data Loggers). The observational depths of the soil water content were at 4,10, 26, 34, 42, 50, 58, 66 and 100 cm. Meteorological data, such as solar radiation, wind speed and air temperature used in ET calculation was obtained from a meteorological station which was set up on the study location.

Equations

The reference crop evapotranspiration (ET0)was estimated by Penmen method (Eq.(1))which was recommended by FAO56 (Allen et al., 1998), and evapotranspiration over the grassland (ET)was estimated by original Penmen-Monteith equation using the observed meteorological data with time unit of 1 h.Moisture availability factor was defined by ma(ET/ET0). Theoretical foundation for describing moisture availability by mawas that it was a function affected by three factors of soil, plant and meteorology. As long as the rate of root absorbing soil moisture could counteract the rate of water loss by transpiration, moisture transfer at a certain rate from soil to atmosphere through the plant could be maintained continuously and plant water potential could be maintained at the normal state, therefore, the soil moisture is in valid state (Guo and Zhang, 1995). The effects of a variety of physiological activities on ET could be detected by normalization (Black, 1979; Kimura et al., 2004).

The height of hypothetical reference crop was 0.12 m, a fixed surface resistance was 70 s · m-1and an albedo was 0.23 (Allen, et al., 1998).

Where, ET0was a reference crop evapotranspiration(mm · h-1); Δ was the slope of the saturation vaporpressure at air temperature (kPa · ℃-1); Rnrepresented the net radiation (MJ · m-2· h-1); γ was the psychometric constant (kPa · ℃-1); G was the soil heat flux(MJ ·m-2· h-1); eswas the saturated vapor-pressure at air temperature (kPa); eawas the actual vapor-pressure(kPa); T was the mean hourly temperature (℃); and u was the average hourly wind speed at height of 2 m (m · s-1).

Actual evapotranspiration (ET)over grassland was calculated by the Penmen-Monteith equation (Eq. (2))which had already been validated in the Liudaogou Catchment (Kimura et al., 2005b, 2007).

Where, lET was the latent heat fl ux (MJ · m-2· h-1); l was the latent heat of vaporization (MJ · kg-1); cpwas the specific heat of air (MJ · kg-1· ℃-1); ρ represented the air density at constant pressure (kg · m-3); rawas the aerodynamic resistance (s · m-1); and rswas the surface resistance (s · m-1), which was calculated by Eq.(3)and was used in calculation of ET in the Liudaogou Catchment (Kimura et al., 2005b).

Where, θ was the soil water content (m3· m-3).

Because ET over grassland generally had impacts on variation of soil water content at the maximum depth of 0.60 m at the study location (Fan, 2005), mean soil water content from the ground surface down to the depth of 0.66 m was used in calculating ET.

Results

The study was conducted in 2006, the average rainfall was 386 mm. Distribution of hourly rainfall, hourly average soil water content (depth: 0-66 cm), ET0, ET and maare shown in Fig. 1A-E, respectively.

The amount of rainfall was 280.2 mm in rainy season from June to September which accounted for 72.6 % of the total rainfall in 2006. Event of the maximum hourly rainfall occurred on June 24th(13:00-15:00 p.m., amount: 45.2 mm)and hourly rainfall reached up to 41.8 mm · h-1. There was no rainfall events whose hourly amount exceeded 10 mm before May and after October in 2006 (Fig. 1A).

Variation of soil water content was sensitive due to rainfall. Soil water content increased significantly due to intensive rainfall and decreased gradually after rainfall ceased. Soil water content remained relatively high in rainy season; however, changed distinctly when intensive rain occurred, and decreased gradually after rainy season. Soil water content changed within a range of 0.064-0.162 cm3· cm-3(Fig. 1B).

ET0was calculated by the assumed full wet condition and it approximately represented potential ET over grassland at a certain climatic condition.As the calculated results of ET0over the Liudaogou Catchment, it showed gradual upward trend since January and maintained relatively high in the rainy season (June-September)and then decreased gradually. In December, ET0fell to nearly the same level as that in January (Fig. 1C). The maximum ET0occurred at 13:00-14:00 p.m. on July 1st and its value was 0.21 mm · h-1. The total ET0in 2006 was 412 mm which was more than the total rainfall (386 mm)and hourly average of ET0throughout 2006 was 0.047 m.

Fig. 1D showed that ET was approximately 0 in January and February and increased gradually since March, it remained at relatively high level in rainy season and after the end of growing season ET presented visible downward trend. In December,ET reduced to a similar level as that in January. ET was significantly affected by intensive rainfall and often suddenly increased to a relatively high level after the intensive rainfall. The main reason was the high temperature combined with the concentrated solar radiation and relatively high soil water content resulted in the strong ET. The maximum ET was 0.14 mm · h-1which occurred at 14:00-15:00 p.m. on June 24th, and daily ET was about 1 mm per day during the rainy season. The total ET in 2006 was 142 mm which was equivalent to 37% of the total rainfall and hourly average of ET throughout 2006 was 0.016 m.

Fig. 1E illustrated that mawas less than 0.4 mm · h-1before May and after November, also most of the time from June to October. ET was lower than ET0for most of the time in the given year. The change of ET was more remarkable than ET0at the time of mawith a high level. The maximum mawas 1.04 which occurred at 15:00 p.m. on June 24th, and it was less than 1.0 during the rest of 2006.

Fig. 1 Hourly distribution of rainfall, soil water content, ET0, ET and ma (2006)

The change processes of ET0, ET and maindicated although the change of mawas affected by both variations of ET0and ET, it was mainly affected by ET, the curve of mawas similar to curve of ET in changeing process. Compared with ET0, maand ET were more sensitive to the intensive rainfall. ET increased significantly after intensive rainfall which led to the obvious change of main the rainy season.

The amount of precipitation, sum of ET0, ET and average of main 2006 are listed in Table 1. Average mawas 0.34 of grassland which indicated that yearly ET was signif i cantly lower than yearly ET0.

Table 1 Precipitation, ET0, ET and ma in 2006

Zhao et al. (2004)pointed out that ET over alfalfa was 2-3 mm per day in summer. Daily ET over grassland (about 1 mm per day)was obviously lower than that of alfalfa in the Liudaogou Catchment.

Conclusions

Based on the analysis results of ET0, ET and main the given year of 2006, conclusions of this study were drawn as the followings:

(1)The amount of ET0was slightly more than the total precipitation and ET over grassland accounted for 37% of total precipitation.

(2)ET increased distinctly after intensive rainfall events and significantly influenced moisture availability factor ma.

(3)Most of mawas less than 0.4 and its annual mean was 0.34.

Allen R G, Pereira L S, Raes D, et al. 1998. Crop evapotranspiration:guidelines for computing crop water requirements. FAO, Rome. pp.300.

Bu Y S, Shao H L, Wang J C. 2002. Effects of different mulching materials on corn seedling growth and soil nutrients contents and distributions. Journal of Soil and Water Conservation, 16(3): 40-42.

Black T A. 1979. Evapotranspiration from douglas fi r stands exposed to soil water def i cits. Water Resources Research, 15: 164-170.

Chen H S, Shao M A, Li Y Y. 2008a. Soil desiccation in the Loess Plateau of China. Geoderma, 143: 91-100.

Chen H S, Shao M A, Li Y Y. 2008b. The characteristics of soil water cycle and water balance on steep grassland under natural and simulated rainfall conditions in the Loess Plateau of China. Journal of Hydrology, 360: 242- 251.

Cheng X R, Huang M B, Shao M A. 2007. Vertical distribution of representative plantation's fi ne root in wind-water erosion crisscross region, Shenmu. Acta Bot Boreal-Occident Sin, 27(2): 321-327.

Fan J. 2005. Study on the soil water dynamics and modeling in waterwind erosion crisscross region on the Loess Plateau. Institute of Soil and Water Conservation, CAS & MWR, Yangling, Shaanxi. China.

Fan J, Shao M A, Wang Q J. 2006. Soil water restoration of alfalfa land in the wind-water erosion crisscross region on the Loess Plateau. Acta Agrestia Sinica, 14(3): 261-264.

Guo Q R, Zhang B G. 1995. Review of studies on soil moisture availability. Tropical and Subtropical Soil Science, 4(2): 119-124.

Han F P, Zheng J Y, Zhang X C. 2009. Plant root system distribution and its effect on soil nutrient on slope land conberted from farmland in the Loess Plateau. Transactions of the CSAE, 25(2): 50-55.

He X B, Li Z B, Hao M D, et al. 2003. Downscale analysis for water scarcity in response to soil-water conservation on Loess Plateau of China. Agriculture, Ecosystems and Environment, 94: 355-361.

Hinikidani O, Huang J B, Yasuda H, et al. 2010. Study on surface runoff characteristics of a small ephemeral catchment in the Northern Loess Plateau, China. Journal of Arid Land Studies, 20(3): 173-178.

Huang M B, Gallichand J, Zhang P C. 2003. Runoff and sediment responses to conservation practices: Loess Plateau of China. Journal of the American Water Resources Association, 39(5): 1197-1208.

Huang J B, Hinokidani O, Kajikawa Y, et al. 2008. Analysis of annual available water resource of a representative basin in upper Loess Plateau. In: Proceeding of the 8th International Conference on Hydro-Science and Engineering. Nagoya, Japan. pp. 129-130.

Huang J B. 2010. Study on runoff and water balance in the northern Loess Plateau, China. The Graduate School of Faculty of Engineering, Tottori University, Japan.

Huo Z, Shao M A, Horton R. 2008. Impact of gully on soil moisture of shrubland in wind-water erosion crisscross region of the Loess Plateau. Pedosphere, 18(5): 674-680.

Jia X X, Shao M A, Wei X R, et al. 2010. State-space simulation of soil surface water content in grassland of northern Loess Plateau.Transactions of the CSAE, 26(10): 38-44.

Kimura R, Okada S, Miura H, et al. 2004. Relationships among the leaf area index, moisture availability, and spectral ref l ectance in an upland rice fi eld. Agricultural Water Management, 69: 83-100.

Kimura R, Liu Y, Takayama N, et al. 2005a. Heat and water balances of the bare soil surface and the potential distribution of vegetation in the Loess Plateau, China. Journal of Arid Environments, 63: 439-457.

Kimura R, Fan J, Zhang X C, et al. 2005b. Evapotranspiration over the grassland fi eld in the Liudaogou basin of the Loess Plateau, China.Acta Oecologica, 29: 45-53.

Kimura R, Bai L, Fan J, et al. 2007. Evapotranspiration estimation over the river basin of the Loess Plateau of China based on remote sensing.Journal of Arid Environments, 68: 53-65.

Li Y S. 2001. Effects of forest on water circle on the Loess Plateau.Journal of Natural Resources, 16(5): 427-432.

Shangguan Z P, Zheng S X. 2006. Ecological properties of soil water and effects on forest vegetation in the Loess Plateau. International Journal of Sustainable Development and World Ecology, 13:307-314.

Sun J Z. 1998. Environmental geology in loess area of China.Environmental Geology and Water Science, 12(1): 49-61.

Unite Nations Environment Programme (UNEP). 1997. World atlas of desertif i cation. Arnold, London. pp. 1-182.

Wang Q, Takahashi H. 1999. A land surface water def i cit model for an arid and semiarid region: impact of desertif i cation on the water def i cit status in the Loess Plateau, China. Journal of Climate, 12: 244-257.

Wang Y Q, Fan J, Shao M A. 2009. Evapotranspiration of three types of plants in water-wind erosion crisscross regions in the Loess Plateau.Acta Aecologica Sinica, 29(10): 5384-5396.

Wu Q X, Yang W Z. 1998. Vegetation construction and sustainable development on the Loess Plateau. Science Press, Beijing. pp. 1-15.

Yang W X. 1996. The preliminary discussion on soil desiccation of artificial vegetation in northern regions of China. Scientia Silvae Sinicae (China), 32: 78-84.

Zhao C, Feng Z, Chen G. 2004. Soil water balance simulation of alfalfa (Medicago sativa L.)in the semiarid Chinese Loess Plateau.Agricultural Water Management, 69: 101-114.

Zheng J Y, Shao M A, Li S Q. 2005. Variation of the hydraulic characteristics of the soil profile in water-wind erosion crisscross region. Transactions of the CSAE, 21(11): 64-66.

Zhu Y J, Shao M A. 2008. Variability and pattern of surface moisture on a small-scale hillslope in Liudaogou Catchment on the northern Loess Plateau of China. Geoderma, 147(3-4): 185-191.