Mapping soil nutrients via different covariates combinations: theory and an example from Morocco

Ecological Processes

Table 3 The semivariogram parameters of studied soil properties via OK (spatial autocorrelation) and KED (with external drifts from bioclimatic data, terrain attributes, and vegetative indices)

Variables	Modeling techniques	Model	Nugget (C₀)	Partial sill (C)	Sill (C₀ + C)	Range (m)	Nugget/Sill	Spatial class	RMSE
pH	OK	Spherical	0.0324	0.0754	0.1078	5517.81	0.301	Moderate	0.283
	KED + C	Spherical	0.0320	0.0755	0.1075	5445.87	0.298	Moderate	0.281
	KED + V	Spherical	0.0324	0.0757	0.1081	5495.52	0.300	Moderate	0.281
	KED + T	Spherical	0.0319	0.0753	0.1072	5463.41	0.298	Moderate	0.284
	KED + A	Spherical	0.0320	0.0731	0.1051	5320.63	0.304	Moderate	0.329
SOM	OK	Spherical	42.400	77.387	119.787	5868.03	0.354	Moderate	9.54
	KED + C	Spherical	42.162	78.233	120.395	5690.85	0.350	Moderate	9.505
	KED + V	Spherical	44.535	76.677	121.212	5707.12	0.367	Moderate	9.653
	KED + T	Spherical	42.488	76.803	119.291	5891.19	0.356	Moderate	9.695
	KED + A	Spherical	43.614	74.576	118.19	5580.74	0.369	Moderate	9.722
K₂O	OK	Spherical	34,448.86	34,961.36	69,410.22	24,678.26	0.496	Moderate	211.79
	KED + C	Spherical	25,794.52	34,949.68	60,744.2	8667.65	0.425	Moderate	211.25
	KED + V	Spherical	31,936.70	32,486.23	64,422.93	12,001.12	0.496	Moderate	211.13
	KED + T	Spherical	32,597.23	33,524.03	66,121.26	17,689.55	0.493	Moderate	209.764
	KED + A	Spherical	27,326.64	33,242.60	60,569.24	7967.91	0.451	Moderate	212.952
P₂O₅	OK	Spherical	981.493	21,708.082	22,689.575	6492.08	0.043	Strong	106.737
	KED + C	Spherical	1024.801	21,660.461	22,685.262	6334.52	0.045	Strong	106.511
	KED + V	Spherical	998.211	21,804.537	22,802.748	6391.46	0.044	Strong	107.70
	KED + T	Spherical	1015.802	21,592.194	22,607.996	6479.01	0.045	Strong	106.894
	KED + A	Spherical	1305.767	21,051.336	22,357.103	6252.80	0.058	Strong	123.91

Bold figures are the best model in each category
Keys: KED +C = kriging with climate covariates as external drift; KED +V = kriging with vegetation covariates as external drift; KED +T = kriging with terrain derivatives as external drift; KED +A = kriging with all covariates as external.