Spatial variability and temporal stability of actual evapotranspiration on a hillslope of the Chinese Loess Plateau
ZHANG Yongkun1, HUANG Mingbin2,*()
1State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining 810016, China 2Institute of Soil and Water Conservation, Northwest Agriculture and Forestry University, Yangling 712100, China
Actual evapotranspiration (ETa) is a key component of water balance. This study aimed to investigate the spatial variability and time stability of ETa along a hillslope and to analyze the key factors that control the spatiotemporal variability of ETa. The potential evaporation, surface runoff and 0-480 cm soil water profile were measured along a 243 m long transect on a hillslope of the Loess Plateau during the normal (2015) and wet (2016) water years. ETa was calculated using water balance equation. Results indicated that increasing precipitation during the wet water year did not alter the spatial pattern of ETa along the hillslope; time stability analysis showed that a location with high time stability of ETa could be used to estimate the mean ETa of the hillslope. Time stability of ETa was positively correlated with elevation (P<0.05), indicating that, on a hillslope in a semi-arid area, elevation was the primary factor influencing the time stability of ETa.
Received: 09 September 2019
Published: 10 March 2021
ZHANG Yongkun, HUANG Mingbin. Spatial variability and temporal stability of actual evapotranspiration on a hillslope of the Chinese Loess Plateau. Journal of Arid Land, 2021, 13(2): 189-204.
Fig. 1Sampling sites in (a) Liudaogou watershed and (b) layout of 38 soil water content monitoring locations spaced at 5 m intervals on a hillslope. Surface runoff experimental plots and evaporation pans at the upper (sampling site, 1-13), middle (sampling site, 14-27), and lower (sampling site, 28-38) slope positions.
Property
Soil profile (cm)
Statistical parameter
Slop position
Upper
Middle
Lower
ELE (m)
Max
1170
1151
1140
Min
1152
1141
1131
Mean±SD
1159±4.72a
1146±3.83b
1136±3.13c
Slope (°)
Max
24.32
16.56
9.21
Min
34.10
25.43
16.23
Mean±SD
27.69±2.90a
22.69±3.09b
12.12±2.07c
Ks (cm/h)
0-30
Max
6.20
7.66
6.31
Min
1.57
1.97
0.89
Mean±SD
3.00±1.52a
3.76±1.78b
2.52±1.65a
BD (g/cm3)
0-30
Max
1.61
1.73
1.74
Min
1.25
1.32
1.13
Mean±SD
1.39±0.09a
1.48±0.1a
1.53±0.11a
LAIm (m2/m2)
Max
1.86
2.32
2.96
Min
0.62
0.61
0.55
Mean±SD
1.13±0.33a
1.56±0.22b
1.47±0.25b
Clay (%)
0-200
Max
16.62
16.26
16.80
Min
9.42
12.33
5.46
Mean±SD
13.56±2.24a
14.70±1.2a
12.97±3.15a
Silt (%)
0-200
Max
40.33
41.71
42.44
Min
26.55
31.58
15.59
Mean±SD
35.26±4.94a
37.20±2.89a
31.57±7.28a
Sand (%)
0-200
Max
64.02
55.65
78.95
Min
43.32
42.69
40.76
Mean±SD
51.19±7.01a
48.10±4.00a
55.46±10.38a
Table 1 Soil properties, topographic attributes, and vegetation features at different slope positions along the hillslope
Fig. 2Distributions of daily precipitation and temperature in the study area during the observation period in 2015 and 2016
Fig. 3Comparisons of potential evapotranspiration (PET) values respectively calculated by evaporation pan and penman equation at the upper slope position on the hillslope
Fig. 4Daily potential evapotranspiration (PET) at the upper, middle, and lower slope positions during the normal (2015, a) and wet (2016, b) water years. Vertical bars represent standard deviations, the same lowercase letters labelled over the boxes indicate no significant difference at P<0.05 level.
Fig. 5Variations in surface runoff depth at different slope positions in 2015 and 2016. Vertical bars represent standard deviations, the same lowercase letters labelled over the boxes indicate no significant difference at P<0.05 level.
Fig. 6Median and interquartile ranges of soil water content (SWC) in the time domains of 2015 (a and c) and 2016 (b and d) for various depths and sampling locations
Fig. 7Variations in accumulated actual evapotranspiration (ETa) in each sampling location along the hillslope during normal (2015) and wet (2016) water years
Parameter
Maximum
Minimum
Mean
SD
Kurtosis
Skewness
MRD (%)
43.12
-20.92
0.00
15.86
1.373
1.265
SDRD (%)
113.40
21.44
42.10
20.24
5.061
2.131
ITS (%)
121.32
22.10
44.32
21.54
5.738
2.287
Table 2 Descriptive statistics of MRD, SDRD, and ITS values in ETa in each sampling location along the hillslope
Fig. 8Ranked mean relative difference (MRD) of actual evapotranspiration (ETa) and the index of time stability (ITS) for each monitoring location on the hillslope. Vertical bars represent ±1 standard deviation of relative difference (SDRD). The dashed curve indicates ITS, and the most time stable location (MTSL) with the lowest ITS values is marked in grey.
Fig. 9Predicted versus measured ETa on the hillslope. The dashed line is the 1:1 line.
MRD
SDRD
ITS
BD
Ks
ELE
LAIm
SWS
SCC
PMCC
MRD
1.000
SDRD
0.787**
1.000
ITS
0.784**
0.996**
1.000
BD
0.066
-0.055
-0.041
1.000
Ks
0.240
0.324*
0.308
0.187
1.000
ELE
0.375*
0.335*
0.331*
-0.543**
0.007
1.000
LAIm
-0.306
-0.309
-0.308
0.426**
0.191
-0.698**
1.000
SWS
0.504**
0.316
0.312
0.029
0.165
0.150
-0.099
1.000
SCC
0.140
0.022
0.032
-0.502**
-0.057
0.466**
-0.316
0.265
1.000
PMCC
0.322*
0.015
0.011
0.239
0.328*
-0.070
0.081
0.370*
0.217
1.000
Table 3 Matrix of Spearman rank correlation coefficients for factors influencing the mean relative difference (MRD), standard deviation of relative difference (SDRD) and index of time stability (ITS) of actual evapotranspiration (ETa)
Parameter
Tmax (°C)
Tmin (°C)
Rs (MJ/m2)
Ws (m/s)
Td (°C)
PET
Pearson correlation
0.816**
0.700**
0.890**
0.185**
0.429**
Sig. (2-tailed)
0.000
0.000
0.000
0.000
0.000
N
1025
1025
1025
1025
1025
Table 4 Matrix of Spearman rank correlation coefficients between potential evapotranspiration (PET) and five climatic factors
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