A spatial downscaling method for SMAP soil moisture considering vegetation memory and spatiotemporal fusion

Figures & data

Figure 1. Map of land cover classifications in Henan province and distribution of 30 in situ stations. SM observation sites are marked with black dotted symbols. The bottom map is obtained based on MCD12Q1.

Table 1. Predictor compositions for the downscaling model.

Sources	Details	Predictors	Spatiotemporal resolution
Soil moisture	ERA5-Land	SM	0.1°/hourly
MODIS	MOD13A3	NDVI	1 km/monthly
	MOD13A2	EVI	1 km/16d
	MCD15A3	LAI	500 m/4d
	MOD11A1	LST	1 km/daily
	MOD16A2	ET	1 km/8d
	MOD16A2	PET	1 km/8d
	MOD17A2	GPP	500 m/8d
	MCD43A3	Albedo	500 m/daily
	MCD12Q1	LandCover	500 m/-
	MOD09GA	Surface reflectance	500 m/ daily
Landsat8	–	Surface reflectance	30 m/16 d
SRTM	–	elevation	30 m/-
	–	slope	30 m/-
	–	aspect	30 m/-
	–	hillshade	30 m/-
SoilGrids	–	sand	250 m/-
	–	clay	250 m/-
		slit	250 m/-
SMAP		TBh	9 km/daily
		TBv	9 km/daily

Figure 2. Flowchart of the SM downscaling method.

Figure 3. Comparison of downscaled SM against SMAP SM for different models. (a) the RF-based model (with NDVI_lagged&NDVI_fused), (b) the RF-based model (without NDVI_lagged), (c) the RF-based model (without NDVI_fused), (d) the polynomial-based model (with NDVI_lagged&NDVI_fused).

Figure 4. Feature importance scores for the downscaling method.

Figure 5. Scatter plots of the original SMAP SM against the in situ SM and downscaled SM of the RF-based model (with NDVI_lagged&NDVI_fused) against the in situ SM at different stations.

Table 2. Detailed statistical metrics for the downscaled SM of the RF-based model (with NDVI_lagged&NDVI_fused) at 30 sites.

Station	R	RMSE (m^3/m^3)	ubRMSE (m^3/m^3)	Bias (m^3/m^3)	a	b
53889	0.653	0.065	0.030	0.057	0.535	0.101
53985	0.636	0.044	0.034	−0.028	0.409	0.079
53989	0.660	0.067	0.048	0.047	0.555	0.100
53993	0.677	0.042	0.029	0.030	0.601	0.072
57066	0.633	0.027	0.027	0.006	0.553	0.071
57067	0.739	0.024	0.019	0.015	0.533	0.073
57099	0.657	0.067	0.040	−0.054	0.758	−0.014
57162	0.681	0.044	0.034	−0.028	0.415	0.073
57177	0.645	0.061	0.046	−0.040	2.022	−0.238
57178	0.628	0.055	0.055	−0.003	0.525	0.055
57187	0.644	0.085	0.056	−0.065	0.549	0.014
57290	0.708	0.047	0.044	0.017	1.471	−0.049
57293	0.641	0.060	0.055	−0.023	3.034	−0.397
57295	0.697	0.061	0.051	0.033	0.946	0.042
57296	0.694	0.058	0.052	0.026	0.691	0.093
58207	0.752	0.077	0.059	0.049	2.655	−0.336
58301	0.738	0.067	0.041	0.052	1.624	−0.057
O2073	0.678	0.043	0.037	0.022	0.540	0.078
O2081	0.661	0.034	0.031	0.015	0.518	0.077
O2176	0.724	0.051	0.030	−0.041	0.455	0.060
O2422	0.636	0.049	0.049	0.006	1.468	−0.055
O2519	0.660	0.059	0.045	−0.038	0.904	−0.017
O4279	0.676	0.051	0.051	−0.004	0.300	0.108
O5291	0.673	0.058	0.037	−0.045	0.605	0.025
501A0500	0.826	0.050	0.048	0.012	1.682	−0.155
501A0700	0.849	0.061	0.042	0.044	1.103	0.018
502A2700	0.713	0.080	0.060	−0.052	1.950	−0.301
502A3500	0.753	0.039	0.037	0.010	0.752	0.069
508A2700	0.686	0.090	0.048	−0.077	2.815	−0.403
620A7800	0.659	0.094	0.040	−0.085	1.268	−0.152
Mean	0.689	0.057	0.043	−0.005	1.075	−0.032

Figure 6. Spatial comparisons of the original SMAP SM and downscaled SM

Figure 7. Temporal variations of original SMAP SM, downscaled SM, in situ SM measurements, and daily precipitation during February 2018 – February 2019 at four selected stations

Table 3. Statistical metrics of the downscaled SM without NDVI_lagged at 30 sites.

Station	R	RMSE (m^3/m^3)	ubRMSE (m^3/m^3)	Bias (m^3/m^3)	a	b
53889	0.480	0.068	0.040	0.056	0.459	0.105
53985	0.224	0.057	0.055	−0.016	0.241	0.116
53989	0.507	0.078	0.058	0.052	0.505	0.112
53993	0.524	0.052	0.039	0.035	0.501	0.089
57066	−0.017	0.060	0.057	0.019	−0.025	0.168
57067	0.215	0.041	0.037	0.018	0.242	0.114
57099	0.353	0.067	0.046	−0.049	0.330	0.064
57162	0.290	0.059	0.058	−0.013	0.331	0.102
57177	0.566	0.053	0.047	−0.024	1.762	−0.171
57178	0.332	0.079	0.075	0.024	0.286	0.109
57187	0.250	0.095	0.084	−0.044	0.232	0.095
57290	0.364	0.057	0.052	0.024	0.752	0.059
57293	0.399	0.045	0.045	−0.002	1.432	−0.081
57295	0.434	0.072	0.062	0.036	0.422	0.122
57296	0.724	0.053	0.051	0.013	0.669	0.083
58207	0.474	0.082	0.078	−0.023	1.890	−0.228
58301	0.582	0.057	0.053	0.023	1.502	−0.066
O2073	0.614	0.055	0.046	0.031	0.684	0.070
O2081	0.477	0.040	0.037	0.016	0.402	0.093
O2176	0.302	0.061	0.053	−0.030	0.305	0.097
O2422	0.349	0.062	0.056	0.027	0.766	0.057
O2519	0.444	0.065	0.054	−0.036	0.585	0.054
O4279	−0.012	0.078	0.078	−0.006	−0.009	0.159
O5291	0.393	0.063	0.055	−0.030	0.462	0.066
501A0500	0.521	0.067	0.058	−0.033	1.008	−0.035
501A0700	0.480	0.078	0.077	−0.011	0.670	0.070
502A2700	0.300	0.094	0.063	−0.070	0.652	0.019
502A3500	0.481	0.071	0.061	−0.037	0.597	0.064
508A2700	0.471	0.069	0.059	−0.036	2.101	−0.234
620A7800	0.414	0.093	0.049	−0.079	0.821	−0.034
Mean	0.398	0.066	0.056	−0.005	0.686	0.041

Table 4. Statistical metrics of the downscaled SM without NDVI_fused at 30 sites.

Station	R	RMSE(m^3/m^3)	ubRMSE(m^3/m^3)	Bias(m^3/m^3)	a	b
53889	0.424	0.091	0.054	0.073	0.603	0.110
53985	0.215	0.071	0.071	0.000	0.340	0.115
53989	0.435	0.105	0.076	0.072	0.592	0.121
53993	0.539	0.071	0.050	0.050	0.786	0.074
57066	−0.057	0.085	0.078	0.033	−0.124	0.196
57067	0.109	0.056	0.051	0.024	0.178	0.128
57099	0.280	0.070	0.048	−0.051	0.260	0.073
57162	0.282	0.077	0.077	0.006	0.472	0.097
57177	0.445	0.062	0.061	−0.013	1.683	−0.145
57178	0.208	0.105	0.097	0.041	0.248	0.131
57187	0.168	0.108	0.105	−0.026	0.211	0.117
57290	0.268	0.087	0.075	0.044	0.780	0.075
57293	0.276	0.067	0.064	0.020	1.370	−0.048
57295	0.379	0.094	0.075	0.056	0.476	0.134
57296	0.686	0.055	0.055	0.008	0.652	0.081
58207	0.432	0.088	0.084	−0.026	1.813	−0.213
58301	0.476	0.059	0.056	0.019	1.244	−0.024
O2073	0.578	0.078	0.062	0.047	0.903	0.059
O2081	0.508	0.051	0.042	0.028	0.599	0.079
O2176	0.267	0.070	0.068	−0.016	0.387	0.096
O2422	0.186	0.092	0.079	0.047	0.553	0.103
O2519	0.319	0.078	0.077	−0.012	0.588	0.078
O4279	−0.107	0.096	0.096	0.010	−0.110	0.191
O5291	0.314	0.076	0.074	−0.015	0.516	0.071
501A0500	0.486	0.071	0.070	−0.010	1.100	−0.034
501A0700	0.379	0.085	0.084	0.008	0.537	0.122
502A2700	0.174	0.096	0.083	−0.048	0.484	0.085
502A3500	0.367	0.076	0.068	−0.035	0.468	0.098
508A2700	0.473	0.087	0.086	−0.012	3.000	−0.371
620A7800	0.288	0.093	0.074	−0.056	0.823	−0.011
Mean	0.327	0.080	0.071	0.009	0.714	0.053

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.