Modeling Long Term Soil Moisture and Evapotranspiration Dynamics in a Temperate Grassland in Inner Mongolia, China

Transcription

1 Modeling Long Term Soil Moisture and Evapotranspiration Dynamics in a Temperate Grassland in Inner Mongolia, China Lu Hao 1, Ge Sun 2, Yongqiang Liu 2, Guangsheng Zhou 3 and Lizhe Yang 1 1. IceMe, Nanjing University of Information Science and Technology, China; 2. USDA Forest Service; 3. Chinese Academy of Sciences

2 The Xilin Gol Steppe is one of the few well-preserved areas of the Inner Mongolia grassland. The overexploitation of water resources in this region has resulted in a series of ecological and environmental issues, including the drying up of rivers and lakes, grassland degradation and ravaging dust storms.

3 Background Concerns of global climate change and land use change in Mongolia Plateau and feedbacks a very sensitive region; Evapotranspiration (ET) is the key component of water balance in arid and semiarid temperate grasslands where water availability is a major limiting factor for ecosystem functions. Understanding temporal variation of ET and soil moisture can help explain the observed environmental changes (i.e., land degradation, ground water decline, climate change): grassland management implications

4 Questions Asked Has the soil moisture changed significantly during past 22 years ( ) at monitoring site, a typical grassland, on the Inner Mongolia Plateau? Could climate change explain the observed changes in soil moisture? Bigger question: Was grasslands degradation caused by climate change or human influences (overgrazing, groundwater over use)?

5 Research Site Xilinhot: National Meteorology Observation Station 44o 08'03"N, 116o19'43"E, Elevation = 990 m Precip = 281 mm/yr. Soil: loam Vegetation: Stipa krylovii Xilinhot Xilin Gol League

6 Methods Soil moisture measurements (10, 20, 50, 70, 100 cm) was observed for 22 years ( ) every 10 days Daily ET measured by a weighing lysimeter ( ) The MIKE SHE, simulates long term water balances

7 MIKE SHE Model FAO ETo ET= f (LAI, SMC, Eto, RootD) Kristensen and Jensen (1975) Richards Equation for unsaturated flow modeling MODIS Leaf Area Index Calibrated with SMC and ET

8 Weighing lysimeter ( daily ET) Soil profile (Soil moisture content and Soil physical parameters) w R 100 % fc R: relative soil moisture, W: soil moisture content (weight based); fc: field capacity W m w md 100% md mwis wet soil weight, md is dry soil weight Effective ET area is 4.0 m2 undisturbed soil columns deep is 2.6 m test accuracy 0.1 mm, the sensitivity is of 0.01 mm.

9 Results 1. Measured Soil Moisture Dynamics. 2. Climatic Variability and Change. 3. MIKE Modeled Long term ET and SMC.

10 10 cm 50 cm 70 cm 100 cm During the past 22 years ( ), soil moisture at both 70 and 100 cm depths dramatically decreased while other two layers showed minimum changes

11 Climate warming and drying (Annual) Increased trend in air temperature; Decreased trend of ppt (especially during the rainy season from July to Sep); More obviously in the last two decades. Continuous droughts in the 2000s. These trends are consistent with those in soil moistures.

12 Climate warming and drying (Seasonal) ( ) M-K test Growing seasons show increased trend in pan evaporation and decreased trend in precipitation (especially from Jul. to Sep.)

13 Rain storm in different periods The lowest days of daily ppt 25mm is 2000s (7d), similar with 1980s; The 1970s is highest for both, very similar with the 1990s.

14 Rain storm: Examples 10-day PPT is ~30 mm, SMC in soil depth less than 50 cm changed (left) 10-day PPT is ~120 mm, SMC in soil depth less than 100 cm changed (right) Water supplement of rain storm to deep soil layers is important.

15 MIKE SHE Results Daily ET ( ) (Nash-Sutcliffe R 2 = 0.73; Correlation Coefficient R = 0.87) Soil moisture content, four soil depths (10, 50, 70, and 100 cm) ( ) (Nash-Sutcliffe R 2 = ; R = )

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17 In the dry year of 2011, precipitation (P, 226mm) was less than the average (282mm) and annual ET was 236 mm, which were higher than P In the wet year of 2012 (512mm), annual ET was 415 mm, less than P

18 Model SMC: 10 cm

19 Model SMC: 50 cm

20 Model SMC: 70 cm

21 Model SMC: 100 cm Soil moisture is difficult to model, but MIKE SHE could capture the pattern from top layers to deep layers

22 Conclusions During the past 22 years ( ), soil moisture at both 70 and 100 cm depths dramatically decreased while other two layers showed minimum changes, suggesting decrease in groundwater recharge; Long-term modeling suggests that during the past 22 years ( ), only half of the times did precipitation meet ET demand, likely causing ecosystem water stress. The recent continuous droughts in the 2000s were the main causes for decrease of soil moisture in subsurface soils, and likely for the decline of groundwater recharge and productivity loss of grasslands.

23 Thank you! Questions?