Shrub encroachment effect on the evapotranspiration and its component—A numerical simulation study of a shrub encroachment grassland in Nei Mongol, China

doi:10.17521/cjpe.2016.0236

Abstract

Abstract:

Aims Shrub encroachment is a common global change phenomenon occurring in arid and semi-arid regions. Due to the difficulty of partitioning evapotranspiration into shrub plants, grass plants and soil in the field, there are few studies focusing on shrub encroachment effect on the evapotranspiration and its component in China. This study aims to illustrate shrub encroachment effect on evapotranspiration by the numerical modeling method. Methods A two-source model was applied and calibrated with the measured evapotranspiration (ET) by the Bowen ratio system to simulate evapotranspiration and its component in a shrub encroachment grassland in Nei Mongol, China. Based on the calibrated model and previous shrub encroachment investigation, we set three scenarios of shrub encroachment characterized by relative shrub coverage of 5%, 15% and 30%, respectively, and quantified their effects caused by shrub encroachment through localized and calibrated two-source model.Important findings The two-source model can well reconstruct the evapotranspiration characteristics of a shrub encroachment grassland. Sensitivity analysis of the model shows that errors for the input variables and parameters have small influence on the result of partitioning evapotranspiration. The result shows that shrub encroachment has relatively small influence on the total amount of ET, but it has clear influence on the proportion of the components of evapotranspiration (E/ET). With shrub coverage increasing from 5% to 15% and then 30%, the evapotranspiration decreased from 182.97 to 180.38 and 176.72 W·m^-2, decreasing amplitude values of 0.34% and 0.44%, respectively. On average, E/ET rises from 52.9% to 53.9% and 55.5%, increasing amplitude values to 2.04% and 3.25%. Data analysis indicates that shrub encroachment results in smaller soil moisture changes, but clear changes of ecosystem structure (decreasing ecosystem leaf area index while increasing vegetation height) which lead to the decrease of transpiration fraction through decreasing canopy conductance. The research highlights that, with the shrub encroachment, more water will be consumed as soil evaporation which is often regarded as invalid part of evapotranspiration and thus resulting in the decrease of water use efficiency.

http://jtp.cnki.net/bilingual/detail/html/ZWSB201703007

Key words: shrub encroachment, eco-hydrological effect, evapotranspiration, two-source model, numerical simulation

Qi-Dan WANG, Wen-Xin YANG, Jie-Yu HUANG, Kun XU, Pei WANG. Shrub encroachment effect on the evapotranspiration and its component—A numerical simulation study of a shrub encroachment grassland in Nei Mongol, China[J]. Chin J Plant Ecol, 2017, 41(3): 348-358.

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Figures/Tables 8

References 32

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输入变量 Input variables	参数 Parameter	参数物理意义 Physical meaning of parameters	单位 Unit
气象数据 Meteorological data	S_d	向下短波辐射 Downward short-wave radiation,	W·m^-2
	h_a	相对湿度 Relative humidity	%
	L_d	向下长波辐射 Downward long-wave radiation	W·m^-2
	P	大气压 Air pressure	hPa
	T_a	气温 Air temperature	℃
	u	水平风速 Horizontal wind speed	m·s^-1
植被属性 Vegetation property	LAI	叶面积指数 Leaf area index	m^-2·m^-2
植被属性 Vegetation property	Zv	植被高度 Vegetation height	m
土壤属性 Soil property	T_soil	土壤热通量测量深度土壤温度 Soil temperature at depth Z_soil	℃
土壤属性 Soil property	θ	根系层土壤含水量 Volumetric soil water content of root layer	m^-3·m^-3
常数 Constant	C_LAI	冠层集聚度 Clumping factor for permittivity of canopy	无量纲 Dimensionless
	θs	土壤饱和含水量 Saturated soil water content	m^-3·m^-3
	r_{st_min}	最小气孔阻抗 Minimum stomata resistance	s·m^-1
	r_{st_max}	最大气孔阻抗 Maximum stomata resistance	s·m^-1
	Z_soil	土壤热通量测量深度 Depth of ground heat flux measurement	m
	α_G	地表反照率 Albedo of ground surface	无量纲 Dimensionless
	α_V	植被冠层反照率 Albedo of vegetation canopy	无量纲 Dimensionless
	λ_ss	土壤表层热传导系数 Thermal conductivity of surface soil	W·m^-1·K^-1
能量通量输出 Output energy flux	σ	斯蒂芬玻尔兹曼常数 Stefan-Boltzmann constant	W·m^-2·K^-4
能量通量输出 Output energy flux	lET	潜热通量 Latent heat flux	W·m^-2
植被冠层输出 Output vegetation canopy layer	T_L	植被冠层温度 Vegetation canopy temperature	℃
植被冠层输出 Output vegetation canopy layer	T	植物蒸散量 Plant evapotranspiration	kg·m^-2·s^-1
土壤层输出 Output of soil layer	T_G	10 cm土壤温度 Soil temperature at 10 cm depth	℃
土壤层输出 Output of soil layer	E	土壤蒸发量 Soil transpiration	kg·m^-2·s^-1

输入变量 Input variables	参数 Parameter	参数物理意义 Physical meaning of parameters	单位 Unit
气象数据 Meteorological data	S_d	向下短波辐射 Downward short-wave radiation,	W·m^-2
	h_a	相对湿度 Relative humidity	%
	L_d	向下长波辐射 Downward long-wave radiation	W·m^-2
	P	大气压 Air pressure	hPa
	T_a	气温 Air temperature	℃
	u	水平风速 Horizontal wind speed	m·s^-1
植被属性 Vegetation property	LAI	叶面积指数 Leaf area index	m^-2·m^-2
植被属性 Vegetation property	Zv	植被高度 Vegetation height	m
土壤属性 Soil property	T_soil	土壤热通量测量深度土壤温度 Soil temperature at depth Z_soil	℃
土壤属性 Soil property	θ	根系层土壤含水量 Volumetric soil water content of root layer	m^-3·m^-3
常数 Constant	C_LAI	冠层集聚度 Clumping factor for permittivity of canopy	无量纲 Dimensionless
	θs	土壤饱和含水量 Saturated soil water content	m^-3·m^-3
	r_{st_min}	最小气孔阻抗 Minimum stomata resistance	s·m^-1
	r_{st_max}	最大气孔阻抗 Maximum stomata resistance	s·m^-1
	Z_soil	土壤热通量测量深度 Depth of ground heat flux measurement	m
	α_G	地表反照率 Albedo of ground surface	无量纲 Dimensionless
	α_V	植被冠层反照率 Albedo of vegetation canopy	无量纲 Dimensionless
	λ_ss	土壤表层热传导系数 Thermal conductivity of surface soil	W·m^-1·K^-1
能量通量输出 Output energy flux	σ	斯蒂芬玻尔兹曼常数 Stefan-Boltzmann constant	W·m^-2·K^-4
能量通量输出 Output energy flux	lET	潜热通量 Latent heat flux	W·m^-2
植被冠层输出 Output vegetation canopy layer	T_L	植被冠层温度 Vegetation canopy temperature	℃
植被冠层输出 Output vegetation canopy layer	T	植物蒸散量 Plant evapotranspiration	kg·m^-2·s^-1
土壤层输出 Output of soil layer	T_G	10 cm土壤温度 Soil temperature at 10 cm depth	℃
土壤层输出 Output of soil layer	E	土壤蒸发量 Soil transpiration	kg·m^-2·s^-1

日期 Date	土壤水观测深度 Depth of observed soil moisture (cm)	灌丛斑块观测土壤体积含水量 Observed multilayer volumetric soil moisture at the shrub patches (%)	草地斑块观测土壤体积含水量 Observed multilayer volumetric soil moisture at the grass patches (%)	灌丛可利用的土壤体积含水量 Available volumetric water content by shrub (%)	草本可利用土壤体积含水量 Available soil volumetric water content by grass (%)	不同灌丛化情景下植被可利用土壤体积含水量 Available volumetric water content by plant under three scenarios of shrub encroachment (%)
日期 Date	土壤水观测深度 Depth of observed soil moisture (cm)					5%盖度 5% coverage	15%盖度 15% coverage	30%盖度 30% coverage
6月16日 June 16th	0-10	30.1	30.0	18.1	22.6	22.42	21.97	21.30
	10-20	22.2	22.6
	20-40	10.0	8.8
	40-60	10.7	9.8
	60-100	17.7	15.5
7月14日 July 14th	0-10	9.3	9.4	11.6	9.0	9.0	9.3	9.7
	10-20	10.2	9.0
	20-40	8.1	8.3
	40-60	13.3	8.1
	60-100	17.0	12.7
8月11日 Aug. 11th	0-10	8.9	7.9	9.7	7.4	7.5	7.8	8.1
	10-20	8.6	7.4
	20-40	6.9	7.3
	40-60	8.8	7.7
	60-100	15.4	11.8
9月13日 Sept. 13th	0-10	8.7	8.8	11.2	8.5	8.6	8.9	9.3
	10-20	8.8	8.5
	20-40	9.1	10.4
	40-60	12.1	12.4
	60-100	17.5	15.9

日期 Date	土壤水观测深度 Depth of observed soil moisture (cm)	灌丛斑块观测土壤体积含水量 Observed multilayer volumetric soil moisture at the shrub patches (%)	草地斑块观测土壤体积含水量 Observed multilayer volumetric soil moisture at the grass patches (%)	灌丛可利用的土壤体积含水量 Available volumetric water content by shrub (%)	草本可利用土壤体积含水量 Available soil volumetric water content by grass (%)	不同灌丛化情景下植被可利用土壤体积含水量 Available volumetric water content by plant under three scenarios of shrub encroachment (%)
日期 Date	土壤水观测深度 Depth of observed soil moisture (cm)					5%盖度 5% coverage	15%盖度 15% coverage	30%盖度 30% coverage
6月16日 June 16th	0-10	30.1	30.0	18.1	22.6	22.42	21.97	21.30
	10-20	22.2	22.6
	20-40	10.0	8.8
	40-60	10.7	9.8
	60-100	17.7	15.5
7月14日 July 14th	0-10	9.3	9.4	11.6	9.0	9.0	9.3	9.7
	10-20	10.2	9.0
	20-40	8.1	8.3
	40-60	13.3	8.1
	60-100	17.0	12.7
8月11日 Aug. 11th	0-10	8.9	7.9	9.7	7.4	7.5	7.8	8.1
	10-20	8.6	7.4
	20-40	6.9	7.3
	40-60	8.8	7.7
	60-100	15.4	11.8
9月13日 Sept. 13th	0-10	8.7	8.8	11.2	8.5	8.6	8.9	9.3
	10-20	8.8	8.5
	20-40	9.1	10.4
	40-60	12.1	12.4
	60-100	17.5	15.9

参数符号 Parameter code	参数名称 Parameter name	lET	T/ET
r_st_{_}_min	最小气孔阻抗 Minimum stomata resistance	-0.28 ± 0.14	-0.08 ± 0.02
r_{st_max}	最大气孔阻抗 Maximum stomata resistance	-0.01 ± 0.08	0.00 ± 0.01
α_V	植被冠层反照率 Albedo of vegetation canopy	-0.17 ± 0.23	-0.02 ± 0.03
α_G	地表反照率 Albedo of ground surface	-0.01 ± 0.06	0.01 ± 0.01
CLAI	冠层集聚度 Clumping factor for permittivity of vegetation	0.02 ± 0.01	0.06 ± 0.03
λ_ss	土壤表层热传导系数 Thermal conductivity of surface soil	-0.05 ± 0.02	0.06 ± 0.02
S_d	向下短波辐射 Downward short-wave radiation,	0.72 ± 0.31	0.00 ± 0.20
L_d	向下长波辐射 Downward long-wave radiation	0.84 ± 0.58	-0.03 ± 0.04
u	水平风速 Horizontal wind speed	0.08 ± 0.15	0.00 ± 0.03
T_a	气温 Air temperature	0.74 ± 0.64	0.21 ± 0.19
h_a	相对湿度 Relative humidity	-1.31 ± 1.02	0.12 ± 0.22
P	大气压 Air pressure	-0.04 ± 0.15	-0.01 ± 0.03
LAI	叶面积指数 Leaf area index	0.42 ± 0.26	0.26 ± 0.25
Z_v	植被高度 Vegetation height	0.23 ± 0.53	0.03 ± 0.09
T_soil	土壤热通量测量深度土壤温度 Soil temperature at depth Z_soil	0.24 ± 0.14	-0.17 ± 0.10
θ	土壤体积含水量 Volumetric soil water content	0.42 ± 0.43	0.12 ± 0.11