A study was carried out to determine soil morphological and chemical properties that influence short‐range variability in the growth of millet [Pennisetum glaucum (L.) R. Br.] in western Niger. Paired productive and adjacent nonproductive sites within millet fields were evaluated along transects in three soil‐geomorphic regions. Transects totaled >300 km in length. Mean depth to subsoil of productive sites ranged from 17.06 cm in the sand valley transect to 19.98 cm in the Dallol Bosso transect. These were different from the topsoil depths of the adjacent nonproductive sites within the respective transects, which ranged from 7.8 to 12.9 cm. Productive sites were located in microtopographic “high” positions relative to nonproductive sites. The mean relief differential between productive and nonproductive sites was 6.13 cm. Vesicular crusts 2 to 5 cm thick and exposed Bt horizon crusts (2.5YR 4/6) were mostly associated with nonproductive sites. The acidity parameters were highly intercorrelated in all transects. Mean pH of soils associated with productive growth of millet ranged from 5.8 to 6.3, compared with 5.4 to 6.0 in the nonproductive areas. Mean exchangeable acidity in the nonproductive sites ranged from 0.2 to 0.36 cmolc kg‐1, about three times higher than the mean in the productive areas. Bray‐1 P levels to a depth of 30 cm did not differ between paired sites in any of the transects. Microtopography was the most important factor influencing millet growth variability in western Niger because it directly influenced the factors related to crust formation and determined the depth to acidic, Al‐rich, P‐deficient subsoil that impeded millet growth and development.

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We evaluate the response of a passive microwave soil moisture retrieval algorithm to errors in the estimation of input variables and parameters. The model is run varying one parameter at a time within a specified range to quantify the effects individual parameters have on soil moisture retrieval. Although errors in the estimation of most parameters yield total variations in soil moisture of less than about 4% volumetric water content (vwc), variations in the estimates of vegetation water content, vegetation b parameter, percent clay, and surface roughness yield the greatest total variations in calculated soil moisture. The effects of these parameter variations on calculated soil moisture are greater for wetter soils (above 25% vwc) and can result in total variations in soil moisture retrieval up to 24% vwc. These same parameters have a compound effect on calculated soil moisture when they vary collectively; variations in soil moisture retrieval with errors in vegetation water content and surface roughness may be as high as 38% vwc (-12%, +26%). Even over more common conditions between 10% and 25% vwc, errors in vegetation water content, percent clay, and surface roughness result in total soil moisture variations of 9% to 15% (±4.5% to which are unacceptably high for many applications. When random errors are imposed on these three parameters of the Southern Great Plains 1997 (SGP97) Hydrology Experiment data set, the macrostructure of the soil moisture distribution remains intact compared to the original calculations, but the moisture field is significantly more heterogeneous. It is demonstrated that the distribution (±2) of soil moisture for given values of brightness temperature ranges between vwc from random errors imposed on the same three parameters. Improvements in parameter estimation in SGP97 contributed to a decrease in the soil moisture uncertainty (a= 0.05) by about 67% to vwc.

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