Monitoring wheat phenology and irrigation in Central Morocco: On the use of relationships between evapotranspiration,crops coefficients,leaf area index and remotely-sensed vegetation indices

The monitoring of crop production and irrigation at a regional scale can be based on the use of ecosystem process models and remote sensing data. The former simulate the time courses of the main biophysical variables which affect crop photosynthesis and water consumption at a fine time step (hourly or daily); the latter allows to provide the spatial distribution of these variables over a region of interest at a time span from 10 days to a month. In this context, this study investigates the feasibility of using the normalised difference vegetation index (NDVI) derived from remote sensing data to provide indirect estimates of: (1) the leaf area index (LAI), which is a key-variable of many crop process models; and (2) crop coefficients, which represent the ratio of actual (AET) to reference (ET₀) evapotranspiration.A first analysis is performed based on a dataset collected at field in an irrigated area of the Haouz plain (region of Marrakesh, Central Morocco) during the 2002–2003 agricultural season. The seasonal courses of NDVI, LAI, AET and ET₀ have been compared, then crop coefficients have been calculated using a method that allows roughly to separate soil evaporation from plant transpiration. This allows to compute the crop basal coefficient (K_cb) restricted to the plant transpiration process. Finally, three relationships have been established. The relationships between LAI and NDVI as well as between LAI and K_cb were found both exponential, with associated errors of 30% and 15%, respectively. Because the NDVI saturates at high LAI values (>4), the use of remotely-sensed data results in poor accuracy of LAI estimates for well-developed canopies. However, this inaccuracy was not found critical for transpiration estimates since AET appears limited to ET₀ for well-developed canopies. As a consequence, the relationship between NDVI and K_cb was found linear and of good accuracy (15%).Based on these relationships, maps of LAI and transpiration requirements have been derived from two Landsat7-ETM+ images acquired at the beginning and the middle of the agricultural season. These maps show the space and time variability in crop development and water requirements over a 3 km × 3 km irrigated area that surrounds the fields of study. They may give an indication on how the water should be distributed over the area of interest in order to improve the efficiency of irrigation. The availability, in the near future, of Earth Observation Systems designed to provide both high spatial resolution (10 m) and frequent revisit (day) would make it feasible to set up such approaches for the operational monitoring of crop phenology and irrigation at a regional scale.     

青海省海东地区草原火灾气象因子特征分析

摘要：利用青海省海东地区1995～2011年17a间草原火灾资料和对应的气象资料,分析研究了海东地区草原火灾地域分布、时空分布及气候等特征,得出了降水、风、气温、地温、相对湿度等气象要素的变化对海东地区草原火灾发生的直接或间接影响,并利用经验公式对高火险区进行划分,区划出海东地区草原火险潜势区,对开展海东地区草原火险预报服务及灾害防御有一定的使用价值。     

4种干旱指标在华北地区气象干旱监测中的适用性分析

采用华北地区1961—2016年69个气象站点常规观测资料，通过选取特殊气象代表站点，选用CI、SPEI、SPI及Z指数分别计算了1、3、12个月时间尺度气象干旱，并与气象干旱发生实况进行对比分析。结果表明：（1）CI计算的华北地区不同区域各时间尺度气象干旱发生频率均较实际明显偏高7%~14%，Z指数计算的发生频率均较实际明显偏低6%~12%，SPI计算的部分区域季时间尺度气象干旱发生频率也明显偏低6%左右；（2）根据华北地区9个代表气象站点计算出的4种干旱指标对比干旱发生实况统计数据，结果显示在4个研究分区中，SPEI指数表征的干旱发生状况与实际吻合率最高，分别为57.41%、60.78%、57.06%和66.99%，与实际干旱发生状况吻合程度最低的是Z指数，4个研究分区的吻合率分别为40.85%、34.94%、41.56%和38.24%。研究表明，SPEI表征各种时间尺度气象干旱发生效果最好，Z指数效果最差。因此，选用SPEI能较准确地表征华北地区气象干旱年际变化及突变检测、时间分布、空间分布、干旱发生强度、不同等级干旱分布等特征。     

气象干旱过程识别及致灾因子特征分析

为科学识别气象干旱及其发生、发展过程，并探究其致灾因子变化特征，通过1981—2020年海西州8个气象站点逐日气象干旱综合指数（Meteorological drought composite index，MCI）描述历年干旱事件，识别干旱过程个数、入旱出旱时间，分析干旱过程强度、干旱过程降水量等。结果表明：1981—2020年，海西州共识别干旱过程192个，干旱开始时间介于5月18日—6月20日之间，结束时间介于7月4日—8月8日之间，历时时长40~63 d；各等级干旱日数变化趋势不一，茫崖、乌兰县干旱日数变化呈增加趋势，变化倾向率分别为2.0 d·（10a）^-1，1.0 d·（10a）^-1，其余站点呈减小趋势，变化倾向率介于-4.0～-1.0 d·（10a）^-1。不同干旱等级的干旱日数大体呈中部少，西部多的态势，西南地区干旱最为严重；时间序列上，轻、中、重、特旱日数和年最长连续干旱日数呈减少趋势；年最长连续干旱日数均值为16 d，最大值为65 d （1995年）；茫崖干旱过程强度最高，天峻和都兰相对较低。     

柴达木地区枸杞种植区遥感提取方法对比研究

枸杞作为柴达木地区特色经济作物之一,利用高分辨率遥感影像开展枸杞种植区识别与提取,有利于政府和农业部门开展市场调控和作物精细化管理。以柴达木典型枸杞种植区诺木洪农场为例,利用随机森林、Softmax、支持向量机、BP神经网络和最大似然5种分类器开展农场内不同生长年限枸杞种植区精细化提取,并对结果进行精度验证。结果表明：采用随机森林的分类效果最佳,其总体分类精度达到93.8%,Kappa 0.93,采用Softmax、支持向量机和BP神经网络方法也均获得了较高的分类精度,其总体分类精度均达到了86.6%~87.6%,Kappa系数达到0.84~0.86,而最大似然法分类效果最差,其总体分类精度仅为76.9%,Kappa系数为0.73。通过实验利用国产高分辨率卫星结合较优的分类器能够实现包括枸杞等小宗特色经济作物种植区域和种植结构的精细化识别和监测。     

利用多条树轮重建黑河上游地区540年径流量

通过黑河上游不同地区的多条树轮年表及该区域降水量、气温和径流量的统计分析,探讨了黑河上游地区树木年轮与径流量的对应关系,重建了黑河上游地区自1450年以来径流量的变化,并进行了变化特征分析,结果表明:径流量的枯水年份稍高。统计得到13个丰水段,累计年份273年,约占总年数的50.6%,相应的枯水段12个,累计年份267年,约占总年数的49.4%,并发现降水量的变化趋势与径流量的变化趋势一致。径流量的中、短周期(45、39、34、3、2a)变化明显。     

青海省玉树地区草原火灾与气象条件关系研究

利用1995年10月~次年5月至2012年10~次年5月玉树地区草原火灾资料和气象观测资料,分析了玉树地区草原火灾的时空分布特征以及降水、风、气温、地温、相对湿度等气象要素的变化对玉树地区草原火灾发生的直接或间接影响,并利用经验公式对玉树地区的草原火险指数进行划分5个气象等级,分别为不易引燃、难燃、可燃、易燃和极易燃,对开展玉树地区草原火险预报服务及灾害防御有一定的使用价值。