The Modification of Meteorological Drought Composite Index and Its Application in Southwest China

The meteorological drought composite index (I_C) is recommended as an effective operational drought monitoring index from National Standard and has been applied by National Climate Center to monitor drought conditions across the whole country in recent years. However, uncontinurous enhancement of drought is frequently observed in the development of drought process at many stations, where uncontinurous enhancement of drought means that daily change is greater than or equal to one drought degree (-0.6) and the I_C value is equal to or less than 0.6 a day before. In order to optimize I_C, modified standard precipitation index (Z₆₅) by weighted average precipitation (W_AP) is introduced. Due to different weighted coefficients of daily precipitation, modified I_C(I_CW) can decrease uncontinurous enhancement of drought to the most extent. Taking Southwest China as an example, daily I_C and I_CW(modified I_C) from 1 January 1961 to 10 May 2010 is calculated, and the differences between I_C and I_CW in frequency of every drought degree and monthly variability of drought frequency are analyzed. Since formula and value range of I_CW are both similar to I_C, the same drought degree is adopted. It indicates that there is no significant difference between I_CW and I_C in frequency of drought. Moreover, the annual intensity of drought from 1961 to 2009 is also analyzed. The results indicate I_CW is also similar to I_C very much and there is no significant trend of annual intensity. Furthermore, as stable index of drought, uncontinurous enhancement of drought of I_C and I_CW are counted. It shows that I_CW has less uncontinurous enhancement of drought than I_C at all four stations, so I_CW has improved the stability of drought according to different weighted coefficients of daily precipitation. Take soil moisture observed on 8, 18 and 28 as representative, the correlation between I_C, I_CW and soil moisture is analyzed from January 1993 to May 2010. The results show that I_CW has greater correlation coefficients with soil moisture of 10 cm at all stations than I_C, and I_CW has greater correlation coefficients with soil moisture of 20 cm at three stations than I_C, showing that I_CW is more consistent with soil moisture.Above all, the analysis shows that I_CW is close to the characteristics of actual drought and suitable for daily drought monitoring in Southwest China. However, there is still some uncontinurous enhancement of drought existing in I_CW due to great uncontinuous change of M₃₀.