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Journal of Arid Land
Journal of Arid Land  2023, Vol. 15 Issue (8): 887-900    DOI: 10.1007/s40333-023-0102-8
Research article     
Dividing the transit wind speeds into intervals as a favorable methodology for analyzing the relationship between wind speed and the aerodynamic impedance of vegetation in semiarid grasslands
LI Ruishen1, PEI Haifeng1, ZHANG Shengwei1,2,3,*(), LI Fengming4, LIN Xi1, WANG Shuai1, YANG Lin1
1College of Water Conservancy and Civil Engineering, Inner Mongolia Agricultural University, Hohhot 010018, China
2Key Laboratory of Water Resources Protection and Utilization of Inner Mongolia Autonomous Region, Hohhot 010018, China
3Autonomous Region Collaborative Innovation Center for Integrated Management of Water Resources and Water Environment in the Inner Mongolia Reaches of the Yellow River, Hohhot 010018, China
4Inner Mongolia Autonomous Region Management Center of Sanshenggong Hydro-junction in the Yellow River, Bayannur 015200, China
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Abstract  

In grassland ecosystems, the aerodynamic roughness (Z0) and frictional wind speed (u*) contribute to the aerodynamic impedance of the grassland canopy. Thus, they are often used in the studies of wind erosion and evapotranspiration. However, the effect of wind speed and grazing measures on the aerodynamic impedance of the grassland canopy has received less analysis. In this study, we monitored wind speeds at multiple heights in grazed and grazing-prohibited grasslands for 1 month in 2021, determined the transit wind speed at 2.0 m height by comparing wind speed differences at the same height in both grasslands, and divided these transit wind speeds at intervals of 2.0 m/s to analyze the effect of the transit wind speed on the relationship among Z0, u*, and wind speed within the grassland canopy. The results showed that dividing the transit wind speeds into intervals has a positive effect on the logarithmic fit of the wind speed profile. After dividing the transit wind speeds into intervals, the wind speed at 0.1 m height (V0.1) gradually decreased with the increase of Z0, exhibiting three distinct stages: a sharp change zone, a steady change zone, and a flat zone; while the overall trend of u* increased first and then decreased with the increase of V0.1. Dividing the transit wind speeds into intervals improved the fitting relationship between Z0 and V0.1 and changed their fitting functions in grazed and grazing-prohibited grasslands. According to the computational fluid dynamic results, we found that the number of tall-stature plants has a more significant effect on windproof capacity than their height. The results of this study contribute to a better understanding of the relationship between wind speed and the aerodynamic impedance of vegetation in grassland environments.

Key words： transit wind speeds      frictional wind speed      aerodynamic roughness      computational fluid dynamic (CFD)      grazed grassland      grazing-prohibited grassland     
Received: 08 October 2022      Published: 31 August 2023
Corresponding Authors: * ZHANG Shengwei (E-mail: zsw@imau.edu.cn)
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LI Ruishen
PEI Haifeng
ZHANG Shengwei
LI Fengming
LIN Xi
WANG Shuai
YANG Lin
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LI Ruishen, PEI Haifeng, ZHANG Shengwei, LI Fengming, LIN Xi, WANG Shuai, YANG Lin. Dividing the transit wind speeds into intervals as a favorable methodology for analyzing the relationship between wind speed and the aerodynamic impedance of vegetation in semiarid grasslands. Journal of Arid Land, 2023, 15(8): 887-900.

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http://jal.xjegi.com/10.1007/s40333-023-0102-8     OR     http://jal.xjegi.com/Y2023/V15/I8/887

Fig. 1 Overview of the study area and experimental apparatus (a) and the images of vegetation growth in grazing-prohibited and grazed grasslands (b)
Fig. 2 Photos of artificial vegetation survey (a), quadrat (b), and wind speed monitor (c)
Plant
species		 Functional group The beginning of the experiment
(15 September 2021)		 The end of the experiment
(15 October 2021)
Grazing-prohibited grassland Grazed grassland Grazing-prohibited grassland Grazed grassland
NPS CPS
(%)		 HPS (cm) NPS CPS
(%)		 HPS (cm) NPS CPS
(%)		 HPS (cm) NPS CPS
(%)		 HPS (cm)
Leymus chinensis (Trin.) Tzvel. TSP 219
±42		 31.6
±3.8		 20.0
±4.7		 91
±22		 15.3
±2.7		 23.6
±6.7		 249
±39		 27.0
±4.3		 23.0
±3.8		 114
±36		 14.6
±6.3		 23.3
±8.8
Cleistogenes squarrosa (Trin.) Keng TSP 17±6 4.6±1.9 11.6±1.8 10±4 1.6±0.3 10.3±3.4 25±8 5.0±2.1 14.0±4.7 11±4 0.6±0.8 10.3±3.3
Stipa krylovii Roshev. TSP 49±18 4.5±1.2 10.2±0.9 21±6 5.3±1.5 12.2±5.3 79±20 17.6±4.5 12.0±3.6 57±19 7.0±2.6 11.0±2.9
Artemisia frigida Willd. SSP - 10.0±3.4 4.2±0.9 - 13.3±2.8 4.2±1.1 - 8.6±2.1 5.0±0.7 - 4.3±1.4 8.3±2.9
Total - 285 30.8 - 122 28.1 - 353 38.9 - 182 30.6 -
Table 1 Vegetation characteristics of grazing-prohibited and grazed grasslands at the beginning and end of the experiment
Fig. 3 Flowchart used in this study for enhancing the relationship between the aerodynamic roughness (Z0) and frictional wind speed (u*)
Fig. 4 Frequency of different transit wind speeds at the wind speed at 0.1 m height (V0.1) in grazing-prohibited grassland (a) and grazed grassland (b)
Grazing measure Transit wind speed (m/s) Wind speed at 2.0 m height (V2.0; m/s) Wind speed at 0.1 m height (V0.1; m/s) A B R2
Grazing-prohibited grassland 0.0-2.0 1.4±0.6 0.4±0.3 -0.12 0.26 0.89
2.0-4.0 3.2±0.5 1.2±0.7* 0.62 -0.07 0.96
4.0-6.0 5.0±0.5 1.9±1.1* 0.96 -0.04 0.98
6.0-8.0 6.8±0.5 3.0±1.2* 0.29 1.24 0.99
8.0-10.0 8.2±0.5 3.9±1.0* 0.21 1.58 0.98
Grazed grassland 0.0-2.0 1.4±0.5 0.4±0.3 -0.41 0.31 0.99
2.0-4.0 3.1±0.5 1.3±0.7* -0.02 0.61 0.99
4.0-6.0 5.0±0.5 2.4±1.1* 0.49 0.84 0.99
6.0-8.0 6.8±0.5 3.8±1.2* 1.46 0.96 0.96
8.0-10.0 8.8±0.5 5.3±0.9* 2.24 3.37 0.87
Table 2 Parameters related to the wind speed profile in grazing-prohibited and grazed grasslands
Fig. 5 (a), relationship between the wind speed at 2.0 m height (V2.0) and Z0; (b-f), relationship between Z0 and V0.1 at the transit wind speeds of 0.0-2.0, 2.0-4.0, 4.0-6.0, 6.0-8.0, and 8.0-10.0 m/s, respectively. For figure b-f, the shaded areas on the right side of y-axis and the upper side of x-axis are the probability densities of V0.1 and Z0, respectively, and the shaded areas inside each figure represent 95% confidence interval.
Transit wind speed (m/s) Grazing-prohibited grassland Grazed grassland
Function expression R2 Function expression R2
0.0-2.0 V0.1=0.852e-0.21Z0 0.58 V0.1= -0.16ln(Z0)+0.655 0.78
2.0-4.0 V0.1=1.970e-0.34Z0 0.73 V0.1=2.057e-0.33Z0 0.82
4.0-6.0 V0.1=3.315e-0.43Z0 0.81 V0.1=3.310e-0.40Z0 0.87
6.0-8.0 V0.1=4.683e-0.44Z0 0.82 V0.1=4.474e-0.38Z0 0.87
8.0-10.0 V0.1=5.609e-0.35Z0 0.76 V0.1=5.643e-0.36Z0 0.74
General V0.1=3.686e-0.42Z0 0.64 V0.1=3.683e-0.43Z0 0.76
Table 3 Z0 as a function of V0.1 for different transit wind speeds
Fig. 6 (a), relationship between V2.0 and frictional wind speed (u*); (b-f), relationship between V0.1 and u* at the transit wind speeds of 0.0-2.0, 2.0-4.0, 4.0-6.0, 6.0-8.0, and 8.0-10.0 m/s, respectively. For figure b-f, in the box plot, the upper and lower limits of the box indicate the 75th and 25th percentile values, respectively; the horizontal lines in each box represent the median; the upper and lower whiskers show the maximum and minimum values, respectively; and the scattered points are outliers. The shaded areas inside each figure represent 95% confidence interval.
Fig. 7 (a), relationship between the ratio of the transit wind speeds to u* (u/u*) and Z0; (b), relationship between Z0 and the average values of u* after dividing the transit wind speeds into intervals (0.0-2.0, 2.0-4.0, 4.0-6.0, 6.0-8.0, and 8.0-10.0 m/s)
Fig. 8 Front view of computational fluid dynamic (CFD) simulation at the transit wind speeds of 2.0, 5.0, and 10.0 m/s in grazing-prohibited grassland (a-c) and grazed grassland (d-f)
Fig. 9 Top view (at 0.1 m height) of CFD simulation at the transit wind speeds of 2.0, 5.0, and 10.0 m/s in grazing-prohibited grassland (a-c) and grazed grassland (d-f). The white points are the position of the geometric model in the wind flow field.
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