Thermal-index monitoring of water status in humid-region urban forests: a canopy-temperature-based approach

Qing Peng; Peng Yin; Xuemin Ye; Hongru Li; Xuan Liang; Zhaofei Tian; Yuexin Xiao; Xiangfen Cheng; Linqi Liu; Kunshui Luo; Wei Liu; Qing Peng; Peng Yin; Xuemin Ye; Hongru Li; Xuan Liang; Zhaofei Tian; Yuexin Xiao; Xiangfen Cheng; Linqi Liu; Kunshui Luo; Wei Liu

doi:10.48130/smartfor-0025-0003

Figures (8) Tables (3)

Figure 1.
The location of (a) the research site, (b) the illustration of flux tower, and (c) the thermal infrared imagery collected from the urban forest ecosystem.
Figure 2.
Seasonal dynamics in (a) soil water content (SWC) and precipitation (P), (b) air temperature (T_a) and net radiation (R_n), (c) wind speed (W_s) and vapor pressure deficit (VPD) in the Nanchang urban forest ecosystem.
Figure 3.
Pearson's correlation coefficients among the canopy-air temperature difference (dT), latent heat flux (LE), and environmental variables under sunny days in the Nanchang urban forest ecosystem. ** represents p < 0.01; * represents p < 0.05.
Figure 4.
Relationships among the soil water content (SWC), vapor pressure deficit (VPD), latent heat flux (LE), and canopy conductance (g_c). ** represents p < 0.01; * represents p < 0.05.
Figure 5.
Canopy-air temperature difference (dT_ll) vs vapour pressure deficit (VPD) under well-watered conditions (all daytime hours data on sunny days were pooled together). ** represents p < 0.01; * represents p < 0.05.
Figure 6.
Performance of the daily empirical plant water stress index (PWSI_e) in indicating urban forest ecosystem water status. ** represents p < 0.01; * represents p < 0.05.
Figure 7.
Daily variations in the daytime mean theoretical plant water stress index (PWSI_{t_day}), canopy conductance (g_c), and latent heat flux (LE) on sunny days from July to October 2024. ** represents p < 0.01; * represents p < 0.05.
Figure 8.
Performance of the daytime mean theoretical plant water stress index (PWSI_{t_day}) in indicating urban forest ecosystem water status. ** represents p < 0.01; * represents p < 0.05.

Time	Slope (°C·kPa⁻¹)	Intercept (°C)	R²
8:00	−0.9226	2.1068	0.8366**
9:00	−0.6772	2.7285	0.5622*
10:00	−0.6640	3.4301	0.2670
11:00	−0.5382	3.7108	0.1008
12:00	−0.2808	3.9867	0.0761
13:00	−0.8012	5.5463	0.5116*
14:00	−0.3989	4.3425	0.1421
15:00	−1.0205	4.4587	0.6430*
16:00	−0.8735	2.6552	0.2374
17:00	−1.5884	2.3692	0.8748**
8:00 − 17:00	−0.8227	3.5879	0.8633**
** represents p < 0.01; * represents p < 0.05 (same in all tables).

Table 1.

Fitted parameters for non-water-stressed baselines.

Time		Slope	Intercept	R²
8:00	g_c	−0.0050	−0.0095	0.0036
8:00	LE	0.0008	−0.2655	0.0278
9:00	g_c	−0.0289	0.5331	0.1609**
9:00	LE	−0.0019	0.7765	0.2080**
10:00	g_c	−0.0595	0.9084	0.2507**
10:00	LE	−0.0037	1.3350	0.2891**
11:00	g_c	−0.0729	1.2223	0.2154**
11:00	LE	−0.0050	1.8831	0.3102**
12:00	g_c	−0.0781	1.0197	0.0282
12:00	LE	−0.0078	2.3512	0.0838*
13:00	g_c	0.0087	−0.2427	0.0040
13:00	LE	0.0020	−0.6967	0.0656
14:00	g_c	0.0253	−0.3908	0.0066
14:00	LE	0.0055	−1.6110	0.0958*
15:00	g_c	0.0043	−0.1196	0.0013
15:00	LE	0.0036	−1.0096	0.2621**
16:00	g_c	−0.0103	0.0991	0.0063
16:00	LE	0.0031	−0.8023	0.1704**
17:00	g_c	−0.0086	0.0288	0.0090
17:00	LE	0.0017	−0.4991	0.1077*

Table 2.

Relationships between empirical plant water stress index (PWSI_e) and canopy conductance (g_c) and between PWSI_e and latent heat flux (LE) at various times.

Time		Slope	Intercept	R²
8:00	g_c	−0.0100	0.2976	0.0320
8:00	LE	−0.0008	0.4058	0.0556
9:00	g_c	−0.0126	0.2601	0.1315**
9:00	LE	−0.0006	0.3169	0.1051*
10:00	g_c	−0.0131	0.2804	0.1995**
10:00	LE	−0.0005	0.3061	0.1060*
11:00	g_c	−0.0081	0.2735	0.1093**
11:00	LE	−0.0002	0.2489	0.0171
12:00	g_c	−0.0129	0.3831	0.2121**
12:00	LE	−0.0003	0.3433	0.0327
13:00	g_c	−0.0131	0.4530	0.2288**
13:00	LE	−0.0005	0.4706	0.1084*
14:00	g_c	−0.0215	0.6037	0.4517**
14:00	LE	−0.0010	0.6855	0.3262**
15:00	g_c	−0.0250	0.7123	0.3034**
15:00	LE	−0.0018	0.9660	0.4931**
16:00	g_c	−0.0221	0.7961	0.1857**
16:00	LE	−0.0022	1.1638	0.5400**
17:00	g_c	−0.0123	0.9101	0.0309
17:00	LE	−0.0025	1.4561	0.3940**

Table 3.

Relationships between theoretical plant water stress index (PWSI_t) and canopy conductance (g_c) and between PWSI_t and latent heat flux (LE) at various times.