Drought Monitoring Technique Based On Land Surface Model And The Study Of Its Application Effects In China

Drought is a recurrent natural disaster. The lack of long-term and large-scale observed soil moisture data limits the drought monitoring and evaluation. Recently, as one of replacement, simulations of land surface model are becoming imported and used more and more. It is still lack of the near-real time drought monitoring technique based on land surface model. Therefore, we use Community Atmosphere Biosphere Land Exchange(CABLE) model to develop the drought monitoring technique on the basis of the simulation results of the model. Through the comparison of definition of SPI(Standardized Precipitation Index), SPEI(Standardized Precipitation Evapotranspiration Index) and PDSI(Palmer Drought Severity Index) and the verification of drought monitoring effects, we study the drought variation in the last sixty years in terms of the trend of drought, drought-effected area, cumulative months, duration and effective mechanism. The following are the main conclusions:(1) Evaluation of model simulation. CABLE-simulated evapotranspiration captures well with results of GSWP-2(Global Soil Wetness Project) model ensemble and observational runoff data. The underestimation of evapotranspiration both in CABLE and GSWP-2 is derived from the forcing precipitation. And CABLE can reproduce the pattern simulated by VIC(Variable Infiltration Capacity) and characteristics illustrated by Tongyu CEOP station. It can also simulate the slowly variation of soil moisture(0~173.8 cm) in central-east China. And the snow melt in spring has obvious effects on soil moisture in northwestern China.(2) Develop a series of near-real time forcing data and complete the drought monitoring technique based on CABLE model. Based on the same method used in developing NCC(NCEP Corrected by CRU) forcing data at the period of 1948-2000, the forcing data was extended to near-real time(seven days late) useing CMAP weekly precipitation replace to CRU precipitation. The data bias, RMSE and correlation between extended forcing and observational data shows the reliability of extended forcing data. A large error has been found in South China and caused by CMAP data. A technique of drought monitoring has been developed by integrating Shell language, NCO(NetCDF Operator) and GrADS(Gridded Analysis Display System). Each process of drought monitoring can be auto-run from forcing data developed to products produced.(3) Assessment of severe drought events happened in recent years in China. Through the comparison of 2001/2002 drought event with observed soil moisture data, the result shows that CABLE can monitor drought processes especially for beginning and ending time and drought classification in Henan province. And the analysis of drought in Southwest China(Guizhou province) shows that the beginning time lags K and CI indices, and duration time is short than two indices. The differences was caused by replenished soil moisture by plenty rainfall in rainy season and K and CI indices reflect meteorological drought.(4) The difference of drought effect derived from definition of drought indices and complexity of physical process was explored through the comparion between model result with SPI, SPEI and PDSI calculated by the same forcing data. The similarity spatial pattern between SPI and SPEI has been illustrated in spring and autumn. It indicates that SPI and SPEI are dominated by precipitation. As temperature has more influence on evaporation than rainfall, a similar spatial pattern has been found both in PDSI and CABLE in spring and autumn. In summer, the different spatial pattern in PDSI and CABLE caused by the dominating effect of energy in the season, the different method of evaporation, vegetation coverage and the different canopy transpiration. The analysis of 1997/1998 and 2009/2010 drought events show that model has the similar effects with PDSI in drought monitoring. And CABLE can also monitor variations of different soil levels and the need of more rainfall and time to replenish vegetation root zone.(5) The evolution characteristics of drought in recent 60 years were explored. A significant drought trend has been found in Northeast, Sorthwest and North China since the middle 1990 s with more droughts. There is a slowly wet trend with fewer droughts in Northwest and Tibetan Plateau. Time periods of the middle 1970 s and around 1990 s are less drought periods. An obvious increase of drought area has been found since 2000. The annual drought area is 35.12% in the period of 1954-2013 in China with 15.01%, 10.02%, 5.02% and 5.07% respectively in less, moderate, severe and extreme drought. There is an obvious increase in Northeast, Southwest and Central China since 2000, and a drought trend in North China since 1995. And the analysis of cumulative drought months illustrates that the biggest increase has been found at the 100°E of northern China. The regions of severe drought from more to less in turn are Northeast, Central China, North China, Southeast, Southwest and Tibeta. And drought happened after 2000 in Northeast, Southeast and Southwest.(6) The dominating factors effecting on drought evolution were explored. It shows that a positive relationship in spring between soil moisture and precipitation with correlation coefficient between 0.44 and 0.67 in many parts of China. And the low temperature in Northeast and Tibeta causes soil freezing, soil porosity decreased, precipitation infiltrating reduced, and with the effect of snow melting, which leads to low correlation between precipitation and soil moisture. And there is a negative relationship between downward radiation and soil moisture in many parts of China. The low temperature and snow cause high albedo value with received energy reduction. The similar result shows in autumn. But a high relationship has been showed in Tibeta and Northeast which is influenced by autumn rain over Southwest China and more rainfall in autumn in Northeast. In summer, the saturated soil moisture and cloudy weather effects of solar radiation energy transmission and absorption are the main reasons why there is a lower relationship in southern than northern China between soil moisture and precipitation and radiation.