RESUMO

Many pedotransfer functions (PTFs) have been developed for predicting the soil water content at different matric potentials. The use of these functions has been encouraged because of the time and work typically required for measuring it, while the PTFs require commonly measured soil properties such as sand, silt, clay, organic matter content, or bulk density for predicting water retention. In addition, several environmental and ecosystem management simulation models such as DRAINMOD, HYDRUS, EPIC, SPAW, and WEPP use PTFs for computing soil hydraulic properties. Because of the increasing use of the PTFs and their effect in many soil water simulation and transport models, this study revised and tested 13 different PTFs for predicting soil water content at -33 and -1500 kPa, values usually known as field capacity and wilting point. Three of these PTFs were derived from tropical soils while the rest were developed with soil samples collected across the United States. These PTFs were evaluated in Chilean soils as an independent dataset and their improvement after calibration was assessed with this new data. The results demonstrate that the PTFs performance depends on the soils used for their development as the estimates showed a significant improvement after calibration. When predicting water content, Rawls et al. (2004) was the best function before calibration (RMSE = 0.08 for -33 and -1500 kPa), while Gupta and Larson (1979) was the best after calibration (RMSE of 0.06 and 0.05, and r2 values of 0.69 and 0.66 at -33 and -1500 kPa, respectively). Nonlinear PTFs performed better than linear PTFs when predicting water content at field capacity. Finally, bulk density proved to be the key variable and can be used as footprint for soils changes through time. Organic matter content was also a significant input but improved the estimates for some specific matric potentials and PTFs.

RESUMO

Soil water content is essential to understand the hydrological cycle. It controls the surface runoff generation, water infiltration, soil evaporation and plant transpiration. This work aims to analyze the spatial distribution of top soil water content and to characterize the spatial mean and standard deviation of top soil water content over time in an experimental catchment located in the Mantiqueira Range region, state of Minas Gerais, Brazil. Measurements of top soil water content were carried out every 15 days, between May/2007 and May/2008. Using time-domain reflectometry (TDR) equipment, 69 points were sampled in the top 0.2 m of the soil profile. Geostatistical procedures were applied in all steps of the study. First, the spatial continuity was evaluated, and the experimental semi-variogram was modeled. For the development of top soil water content maps over time a co-kriging procedure was used having the slope as a secondary variable. Rainfall regime controlled the top soil water content during the wet season. Land use was also another fundamental local factor. The spatial standard deviation had low values under dry conditions, and high values under wet conditions. Thus, more variability occurs under wet conditions.

A umidade do solo é essencial para o entendimento do ciclo hidrológico, uma vez que controla a geração do escoamento superficial, infiltração de água no solo, evaporação do solo e transpiração das plantas. Este trabalho objetivou analisar os padrões espaciais da umidade superficial do solo e caracterizar a média e o desvio padrão espaciais da mesma ao longo do tempo em uma bacia hidrográfica experimental localizada na Serra da Mantiqueira, MG. As medidas da umidade superficial do solo foram conduzidas a cada 15 dias, entre Maio/2007 e Maio/2008, usando um equipamento TDR portátil, em 69 pontos amostrados na camada de 0-20 cm. Procedimentos geoestatísticos foram aplicados em todas as etapas do trabalho. Primeiramente, a continuidade espacial foi avaliada modelando-se o semivariograma experimental. Mapas de umidade do solo foram desenvolvidos com base na cokrigagem usando o padrão de declividade como variável secundária. O regime de chuvas controlou a umidade superficial do solo durante o período úmido, devido aos altos conteúdos de umidade. O uso do solo também foi outro fator fundamental na distribuição espacial da umidade do solo. O desvio padrão espacial apresentou baixos valores sob condições secas e valores mais altos sob condições úmidas.

RESUMO

Soil water content is essential to understand the hydrological cycle. It controls the surface runoff generation, water infiltration, soil evaporation and plant transpiration. This work aims to analyze the spatial distribution of top soil water content and to characterize the spatial mean and standard deviation of top soil water content over time in an experimental catchment located in the Mantiqueira Range region, state of Minas Gerais, Brazil. Measurements of top soil water content were carried out every 15 days, between May/2007 and May/2008. Using time-domain reflectometry (TDR) equipment, 69 points were sampled in the top 0.2 m of the soil profile. Geostatistical procedures were applied in all steps of the study. First, the spatial continuity was evaluated, and the experimental semi-variogram was modeled. For the development of top soil water content maps over time a co-kriging procedure was used having the slope as a secondary variable. Rainfall regime controlled the top soil water content during the wet season. Land use was also another fundamental local factor. The spatial standard deviation had low values under dry conditions, and high values under wet conditions. Thus, more variability occurs under wet conditions.

A umidade do solo é essencial para o entendimento do ciclo hidrológico, uma vez que controla a geração do escoamento superficial, infiltração de água no solo, evaporação do solo e transpiração das plantas. Este trabalho objetivou analisar os padrões espaciais da umidade superficial do solo e caracterizar a média e o desvio padrão espaciais da mesma ao longo do tempo em uma bacia hidrográfica experimental localizada na Serra da Mantiqueira, MG. As medidas da umidade superficial do solo foram conduzidas a cada 15 dias, entre Maio/2007 e Maio/2008, usando um equipamento TDR portátil, em 69 pontos amostrados na camada de 0-20 cm. Procedimentos geoestatísticos foram aplicados em todas as etapas do trabalho. Primeiramente, a continuidade espacial foi avaliada modelando-se o semivariograma experimental. Mapas de umidade do solo foram desenvolvidos com base na cokrigagem usando o padrão de declividade como variável secundária. O regime de chuvas controlou a umidade superficial do solo durante o período úmido, devido aos altos conteúdos de umidade. O uso do solo também foi outro fator fundamental na distribuição espacial da umidade do solo. O desvio padrão espacial apresentou baixos valores sob condições secas e valores mais altos sob condições úmidas.