Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Sensors (Basel). 2019 Jan 21;19(2):430. doi: 10.3390/s19020430.
Identification of drought is essential for many environmental and agricultural applications. To further understand drought, this study presented spatial and temporal variations of drought based on satellite derived Vegetation Condition Index (VCI) on annual (Jan⁻Dec), seasonal monsoon (Jun⁻Nov) and pre-monsoon (Mar⁻May) scales from 1982⁻2015 in Nepal. The Vegetation Condition Index (VCI) obtained from NOAA, AVHRR (National Oceanic and Atmospheric Administration, Advanced Very High Resolution Radiometer) and climate data from meteorological stations were used. VCI was used to grade the drought, and the Mann⁻Kendall test and linear trend analysis were conducted to examine drought trends and the Pearson correlation between VCI and climatic factors (i.e., temperature and precipitation) was also acquired. The results identified that severe drought was identified in 1982, 1984, 1985 and 2000 on all time scales. However, VCI has increased at the rate of 1.14 yr ( = 0.04), 1.31 yr ( = 0.03) and 0.77 yr ( = 0.77) on the annual, seasonal monsoon and pre-monsoon scales, respectively. These increased VCIs indicated decreases in drought. However, spatially, increased trends of drought were also found in some regions in Nepal. For instance, northern areas mainly in the Trans-Himalayan regions identified severe drought. The foothills and the lowlands of Terai (southern Nepal) experienced normal VCI, i.e., no drought. Similarly, the Anomaly Vegetation Condition Index (AVCI) was mostly negative before 2000 which indicated deficient soil moisture. The exceedance probability analysis results on the annual time scale showed that there was a 20% chance of occurring severe drought (VCI ≤ 35%) and a 35% chance of occurring normal drought (35% ≤ VCI ≤ 50%) in Nepal. Drought was also linked with climates in which temperature on the annual and seasonal monsoon scales was significant and positively correlated with VCI. Drought occurrence and trends in Nepal need to be further studied for comprehensive information and understanding.
识别干旱对于许多环境和农业应用至关重要。为了进一步了解干旱，本研究基于卫星衍生的植被状况指数（VCI），在 1982 年至 2015 年期间，在尼泊尔对年（1 月至 12 月）、季风季节（6 月至 11 月）和前季风（3 月至 5 月）尺度上进行了干旱的时空变化研究。本研究使用了来自美国国家海洋和大气管理局（NOAA）、高级甚高分辨率辐射计（AVHRR）的 VCI 和气象站的气候数据。VCI 用于对干旱进行分级，进行了 Mann-Kendall 检验和线性趋势分析，以检验干旱趋势，并获得了 VCI 与气候因素（即温度和降水）之间的 Pearson 相关关系。结果表明，在所有时间尺度上，1982 年、1984 年、1985 年和 2000 年均发生了严重干旱。然而，VCI 在年、季风季节和前季风季节的增长率分别为 1.14 年（=0.04）、1.31 年（=0.03）和 0.77 年（=0.77）。这些增加的 VCI 表明干旱程度有所降低。然而，从空间上看，尼泊尔的一些地区也出现了干旱加剧的趋势。例如，北部主要在喜马拉雅山地区的地区发生了严重干旱。特莱低地（尼泊尔南部）的山麓和低地的 VCI 则处于正常状态，即没有干旱。同样，2000 年之前，异常植被状况指数（AVCI）主要为负，这表明土壤水分不足。年时间尺度上的超越概率分析结果表明，尼泊尔发生严重干旱（VCI≤35%）的概率为 20%，发生正常干旱（35%≤VCI≤50%）的概率为 35%。干旱还与气候有关，年和季风季节的气温与 VCI 呈显著正相关。需要进一步研究尼泊尔的干旱发生和趋势，以获取更全面的信息和认识。
