CN115266648A - 一种二类水体固有光学参数优化模拟方法 - Google Patents

一种二类水体固有光学参数优化模拟方法 Download PDF

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CN115266648A
CN115266648A CN202210712267.8A CN202210712267A CN115266648A CN 115266648 A CN115266648 A CN 115266648A CN 202210712267 A CN202210712267 A CN 202210712267A CN 115266648 A CN115266648 A CN 115266648A
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赵利民
陈瀚阅
丁月圆
李家国
陈兴峰
陈洪真
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Abstract

针对当前水色三要素遥感监测缺乏多组分水色要素同步反演方法、存在信息多散、汇集复杂、分析获取不及时等问题,本发明公开了一种二类水体固有光学参数优化模拟方法,该方法包括如下步骤:步骤1)选取研究区并设计试验方案进行采样点处叶绿素a浓度、悬浮物浓度、黄色物质在440nm处的吸收系数以及水面光谱测量;步骤2)计算各采样点的吸收系数和后向散射系数;步骤3)利用2SeaColor辐射传输模型模拟各采样点遥感反射率;步骤4)计算模拟出的遥感反射率和实测采样点的遥感反射率均方根差并确定研究区固有光学参数值。

Description

一种二类水体固有光学参数优化模拟方法
技术领域
本发明涉及一种二类水体固有光学参数优化模拟方法,面向我国典型的内陆二类湖泊太湖,基于野外实地采样数据以及2SeaColor模型优化模拟得到的研究区固有光学参数,实现水色要素高精度同步反演。
背景技术
水资源短缺、水生态损害、水环境污染等新问题日益突出,基于“三水统筹”的水生态安全管理面临新的挑战。水生态监测是水生态环境管理的“顶梁柱”,是生态文明建设的重要基础支撑之一。生态环境部发布的《关于开展生态环境遥感监测试点工作的通知》(监测函〔2019〕6号)要求试点省份加快构建生态环境遥感调查、监测与评估体系。《重点流域水生态环境保护规划(2021-2025年) (征求意见稿)》明确指出,要基于流域特色要素,制定统筹水资源、水生态、水环境的指标体系,以高水平保护引导推动高质量发展。
地表水体分为一类水体和二类水体。大洋开阔水体主要为一类水体,近岸河口水体主要为二类水体。由于二类水体更接近人类生产生活区,所以受人类活动影响更大,污染严重。近年来,我国多个湖泊出现富营养化、有机污染、水域面积缩小、盐渍化、生态系统失衡等问题(程琦,2012)。水体富营养化不但会破坏湖泊的整体功能,破坏湖泊生态系统的多样性,而且还会危害浮游植物,导致水华暴发。藻类产生的毒素通过饮用水厂影响人类健康,威胁人类生存,水面上的藻类散发出异味,严重影响河湖景观。
长期以来,近岸二类水体水色三要素的反演一直是水色光学遥感中的一个难题。水色三要素主要包括叶绿素(Chlorophyll,以Chlorophyll-a为主Chl-a)、非藻类固体悬浮物(Suspended Particulate Matter,SPM)和有色可溶性有机物 (ChromophoricDissolved Organic Matter or ColoredDissolved Organic Material, CDOM)(唐军武等,2003)。目前水色三要素的遥感监测方法主要分为经验方法、半经验/半分析方法和分析方法三种,从辐射传输角度进行反演的模型很少,往往以单组分水色要素反演模型为主,多组分水色要素同步反演模型少。
鉴于目前二类水体水色三要素遥感监测方法存在的不足,本发明将研究焦点集中在实测的叶绿素a浓度、悬浮物浓度、黄色物质在440nm处的吸收系数和水面光谱的关系上,通过水色三要素与遥感反射率建立数学关系,提出一种二类水体固有光学参数优化模拟方法,进而有效地弥补传统监测方法在时间和空间尺度上的不连续性,提供连续、准确的水质参数变化数据。
发明内容
针对当前水色三要素遥感监测缺乏多组分水色要素同步反演方法、存在信息多散、汇集复杂、分析获取不及时等问题,本发明提出了一种二类水体固有光学参数优化模拟方法,为水生态保护提供可靠有效准确的信息源。
本发明的目的通过以下技术步骤实现:
步骤1)选取研究区并设计试验方案进行采样点处叶绿素a浓度、悬浮物浓度、黄色物质在440nm处的吸收系数以及水面光谱测量;
步骤2)计算各采样点的吸收系数和后向散射系数;
步骤3)利用2SeaColor辐射传输模型模拟各采样点遥感反射率;
步骤4)计算模拟出的遥感反射率和实测采样点的遥感反射率均方根差并确定研究区固有光学参数值。
所述步骤1)的具体方法为:
a)选取研究区域;b)基于研究区设计湖面试验方案;c)水质参数实地测量。
所述步骤2)的具体方法为:
a)计算研究区各采样点的吸收系数:
a(λ)=aW(λ)+aChla(λ)+as(λ)+aCDOM(λ)
式中,a(λ)指波长λ处的水体总吸收系数;aW(λ)为水分子吸收系数、aChla(λ)为叶绿素a吸收系数、as(λ)为悬浮物的吸收系数、aCDOM(λ)为黄色物质所产生的吸收系数,单位都是每米(m-1)。
b)计算研究区各采样点的后向散射系数:
bb(λ)=bw(λ)+bchla(λ)+bs(λ)
式中,bb(λ)指波长λ处的水体总散射系数;bw(λ)水分子散射系数;bChla(λ)为叶绿素a散射系数;bs(λ)为悬浮物的散射系数。
所述步骤3)的具体方法为:
a)将研究区的固有光学参数叶绿素a浓度的指数项x、悬浮物散射光谱斜率ns、和悬浮物在550nm处的散射系数bs(550)三个要素作为未知量;b)将三个未知量设定合理的初值、范围和步长,利用2SeaColor模型不断循环得到对应于每一组x、ns、和bs(550)的遥感反射率。
其中,2SeaColor模型的理论公式为(Salama等,2015):
Figure BDA0003708516170000031
Figure BDA0003708516170000032
μw=cosθ′s
Figure BDA0003708516170000033
Figure BDA0003708516170000034
式中,
Figure BDA0003708516170000035
是半无限介质的定向半球面反射率;
x是水体总后向散射系数bb(单位m-1)和总吸收系数a的比值(单位m-1);
μw指平坦水面下太阳天顶角的余弦值;若水面上太阳天顶角为θs,则θ′s=arcsin(sinθs/nw),其中nw是水的折射系数,取值1.33;
R(0-)是水平面下辐亮度反射率;
Q是水平面向上的辐亮度反射率与水平面向下的辐亮度反射率的比值,取值3.25;
Rrs为离水反射率。
所述步骤4)的具体方法为:
a)计算每一个采样点模拟得到的遥感反射率与同步实测的遥感反射率的均方根差(RMSE);b)找出最小的RMSE,确定研究区的固有光学参数值。
附图说明
图1为本发明实施的方法流程示意图;
图2为试验采样点分布图;
图3为模拟出的各采样点的遥感反射率图。
具体实施方式
下面结合附图对本发明“一种二类水体固有光学参数优化模拟方法”作进一步阐述说明。
(一)试验设计与水体样本采集
首先,选取研究区域(以近岸二类水体太湖为例);其次,在研究区范围内设计覆盖整个湖面的试验采样点,如图2所示。在太湖北部梅梁湾、竺山湾等水污染较严重区域加密采样点,在太湖南部较清洁水体区域适当减少采样点;最后,采用水样瓶进行水体采样用于实验室测量叶绿素a浓度、悬浮物浓度和黄色物质在440nm处的吸收系数以及采用Fieldspec HandHeld2手持便携式光谱分析仪进行光谱数据采集。
(二)吸收系数和散射系数的计算
水体中的吸收系数考虑水体中的四种组分,即水分子、叶绿素a、悬浮物和黄色物质,其中纯水分子的吸收系数是波长λ的函数。基于研究区采样点实测的叶绿素a浓度、悬浮物浓度以及440nm处黄色物质的吸收系数,可以求得研究区总吸收系数。
其中,叶绿素a的吸收系数的计算公式为:
Figure BDA0003708516170000041
aChla(λ)=(a0(λ)+a1(λ)ln aChla(440))×aChla(440)
式中,Cchla为叶绿素a浓度,a0(λ)、a1(λ)为经验参数,achl(440)为太湖叶绿素a波长440nm处的光谱吸收系数。
悬浮物的吸收系数的计算公式为:
Figure BDA0003708516170000042
式中,CS为悬浮物浓度,as(440)为太湖悬浮物波长440nm处的光谱吸收系数,
Figure BDA0003708516170000043
为悬浮物的归一化单位吸收系数。
黄色物质的吸收系数的计算公式为:
aCDOM(λ)=aCDOM(440)exp[-SCDOM(λ-440)]
式中,SCDOM为黄色物质吸收光谱斜率,本研究根据实测数据确定的平均值为0.015nm-1
由于黄色物质对光的后向散射作用非常微弱,因此后向散射系数的计算只考虑水体中的三种组分,即水分子、叶绿素和悬浮物,其中纯水分子的散射系数是波长λ的函数。
其中,叶绿素a的散射系数的计算公式为:
Figure BDA0003708516170000044
式中,
Figure BDA0003708516170000045
为叶绿素a的归一化单位吸收系数,x为叶绿素a浓度的指数项,其优化范围为(0.01,2.00),步长为0.01。
悬浮物的散射系数的计算公式为:
Figure BDA0003708516170000046
式中,bs(550)为悬浮物550nm处的散射系数,设置其优化范围为(0.01,2.00),步长为0.01;ns为悬浮物散射光谱斜率,设置其优化范围为(-1.00,1.00),步长为0.01。
将x、ns、和bs(550)三个研究区固有光学参数作为未知数,通过设置的初始值、范围和步长的情况下不断进行循环计算,得到与x、ns、和bs(550)相对应的各个采样点总散射系数的值。
(三)遥感反射率模拟结果与固有光学参数确定
基于步骤(二)中计算的吸收系数、散射系数以及本发明基于2SeaColor 模型模拟得到的400-700nm波长范围的遥感反射率与实测遥感反射率进行均方根差计算。在均方根误差达到最小时,确定对应的x为0.41、ns为-1.15、bs(550) 为0.03即为适合研究区水色三要素反演的固有光学参数值。以波长为横轴,模拟得到的遥感反射率为纵轴作遥感反射率随波长变化曲线图,选取代表性采样点数据成图,如图3所示;
其中,均方根差的计算公式为:
Figure BDA0003708516170000051
式中,i为遥感反射率波段数,
Figure BDA0003708516170000052
为模拟出的遥感反射率,
Figure BDA0003708516170000053
为实测的遥感反射率。

Claims (5)

1.一种二类水体固有光学参数优化模拟方法,该方法包括以下步骤:
步骤1)选取研究区并设计试验方案进行采样点处叶绿素a浓度、悬浮物浓度、黄色物质在440nm处的吸收系数以及水面光谱测量;
步骤2)计算各采样点的吸收系数和后向散射系数;
步骤3)利用2SeaColor辐射传输模型模拟各采样点遥感反射率;
步骤4)计算模拟出的遥感反射率和实测采样点的遥感反射率均方根差并确定研究区固有光学参数值。
2.如权利要求书1所述的方法,其特征在于,所述步骤1):a)选取研究区域;b)基于研究区设计湖面试验方案;c)水质参数实地测量。
3.如权利要求书1所述的方法,其特征在于,所述步骤2):a)计算研究区各采样点的吸收系数;b)计算研究区各采样点的后向散射系数:
a(λ)=aW(λ)+aChla(λ)+as(λ)+aCDOM(λ) (1)
式(1)中,a(λ)指波长λ处的水体总吸收系数;aW(λ)为水分子吸收系数、aChla(λ)为叶绿素a吸收系数、as(λ)为悬浮物的吸收系数、aCDOM(λ)为黄色物质所产生的吸收系数,单位都是每米(m-1)。
bb(λ)=bw(λ)+bchla(λ)+bs(λ) (2)
式(2)中,bb(λ)指波长λ处的水体总散射系数;bw(λ)水分子散射系数;bChla(λ)为叶绿素a散射系数;bs(λ)为悬浮物的散射系数。
4.如权利要求书1所述的方法,其特征在于,所述步骤3):a)将研究区的固有光学参数叶绿素a浓度的指数项x、悬浮物散射光谱斜率ns、和悬浮物在550nm处的散射系数bs(550)三个要素作为未知量;b)将三个未知量设定合理的初值、范围和步长,利用2SeaColor模型不断循环得到对应于每一组x、ns、和bs(550)的遥感反射率。2SeaColor模型的理论公式如下:
Figure FDA0003708516160000011
Figure FDA0003708516160000012
μw=cosθ′s
Figure FDA0003708516160000013
Figure FDA0003708516160000021
式(3)中,
Figure FDA0003708516160000022
是半无限介质的定向半球面反射率;x是水体总后向散射系数bb(单位m-1)和总吸收系数a的比值(单位m-1);μw指平坦水面下太阳天顶角的余弦值;若水面上太阳天顶角为θs,则θ′s=arcsin(sinθs/nw),其中nw是水的折射系数,取值1.33;R(0-)是水平面下辐亮度反射率;Q是水平面向上的辐亮度反射率与水平面向下的辐亮度反射率的比值,取值3.25;Rrs为离水反射率。
5.如权利要求书1所述的方法,其特征在于,所述步骤4):a)计算每一个采样点模拟得到的遥感反射率与同步实测的遥感反射率的均方根差(RMSE);b)找出最小的RMSE,确定研究区的固有光学参数值。
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