Revisiting Field Capacity (FC): variation of definition of FC and its estimation from pedotransfer functions

Abstract

SUMMARYTaking into account the nature of the hydrological processes involved in in situ measurement of Field Capacity (FC), this study proposes a variation of the definition of FC aiming not only at minimizing the inadequacies of its determination, but also at maintaining its original, practical meaning. Analysis of FC data for 22 Brazilian soils and additional FC data from the literature, all measured according to the proposed definition, which is based on a 48-h drainage time after infiltration by shallow ponding, indicates a weak dependency on the amount of infiltrated water, antecedent moisture level, soil morphology, and the level of the groundwater table, but a strong dependency on basic soil properties. The dependence on basic soil properties allowed determination of FC of the 22 soil profiles by pedotransfer functions (PTFs) using the input variables usually adopted in prediction of soil water retention. Among the input variables, soil moisture content θ θ θ θ θ (6 kPa) had the greatest impact. Indeed, a linear PTF based only on it resulted in an FC with a root mean squared residue less than 0.04 m 3 m -3 for most soils individually. Such a PTF proved to be a better FC predictor than the traditional method of using moisture content at an arbitrary suction. Our FC data were compatible with an equivalent and broader USA database found in the literature, mainly for medium-texture soil samples. One reason for differences between FCs of the two data sets of finetextured soils is due to their different drainage times. Thus, a standardized procedure for in situ determination of FC is recommended.Index terms: internal drainage, aeration capacity, soil water retention. Levando em conta a natureza dos processos hidrológicos envolvidos na medição da Capacidade de Campo (CC) in situ, este estudo propõe uma variação da definição de CC a fim de minimizar as impropriedades de sua determinação, mas também de manter seu sentido prático original. A análise de dados de CC para 22 solos brasileiros e de dados adicionais da literatura, todos medidos segundo a definição proposta, que é com base no tempo de drenagem de 48 h após uma infiltração por alagamento raso, indicou fraca dependência na quantidade de água infiltrada, valor de umidade antecedente, morfologia do solo e nível do lençol freático, mas forte dependência nas propriedades básicas do solo. Essa dependência nas propriedades básicas do solo permitiu a determinação da CC dos 22 perfis de solo por funções de pedotransferência (FPTs), utilizando as variáveis de entrada usualmente adotadas na predição de retenção de água no solo. Entre as variáveis de entrada, a umidade θ (6 kPa) foi a que teve maior impacto; de fato, uma FPT linear com base somente nela resultou numa CC com raiz quadrada de resíduo quadrático médio menor que 0,04 m³ m -³, individualmente para todos os solos. Foi evidenciado que tal FPT foi um melhor avaliador da CC do que o método tradicional que utiliza diretamente a umidade a uma sucção arbitrária. Os dados de CC foram compatíveis com uma base de ...

Variation of soil hydraulic properties with alpine grassland degradation in the eastern Tibetan Plateau

“…Wei et al (2010) noted that CP changed similarly with degradation, i.e. CP was positively correlated with FC, which was consistent with the definition of FC (Ottoni Filho et al, 2014). K s decreased initially and then increased with degradation ( Fig.…”

“…Moreover, a positive correlation between CP and SOC was detected in many studies (Gao et al, 2015;Price et al, 2010;Yu et al, 2015), reflecting that CP can be an indicator of soil quality. By definition, FC is the maximum water content held in soils when excess water has drained away and the downward flux is negligible (Ottoni Filho et al, 2014). Therefore, FC is essentially dependent on the capillary effect; decreased FC is the result of a decrease in CP.…”

“…FC and K s are mainly influenced by vegetation, soil (Pachepsky and Park, 2015), topography (Leij et al, 2004), climate (Jarvis et al, 2013) and human activities (Mubarak et al, 2009;Palese et al, 2014). In recent years, vegetation degradation has been widespread because of natural environmental changes and anthropogenic influences.…”

An Algebraic Pedotransfer Function to Calculate Standardized in situ Determined Field Capacity

“…Additionally, the concept of "free drainage" is accepted to be vague, "negligible drainage" to be undefined and that two-to-three-day drainage time might not be compatible with negligible drainage (Cassel & Nielsen, 1986;Hillel, 1998;Romano & Santini, 2002;Twarakavi, Sakai, & Simunek, 2009;Assouline & Or, 2014). Due to operating field experiment difficulties, FC is commonly estimated in laboratory from soil samples, such as the water content at a specific suction value, typically 10 kPa or 33 kPa, which is generally considered incompatible with the definition of FC (Hillel, 1998;Twarakavi et al, 2009;Nemes, Pachepsky, & Timlin, 2011;Romano, Palladino, & Chirico, 2011;Ottoni Filho, Ottoni, Oliveira, Macedo, & Reichardt, 2014a). Thus, it is not only the FC concept that is questionable (Cassel & Nielsen, 1986;Assouline & Or, 2014), but also its most usual determination methodology.…”

“…These are the meanings that lead to a great practical demand for FC. Ottoni Filho et al (2014a) redefined FC as being "the vertical distribution of the volumetric water content in the upper part of a soil profile that, in the course of ponded infiltration (of water from any source and with ponding depth smaller than 10 cm), becomes fully wetted at the end of infiltration and remains exposed to the subsequent process of drainage without evapotranspiration or rain for 48 h". The text considers that FC is a moisture profile, FC(z), and is consistent with the fact the FC concept must be based on hydraulic processes that occur in actual soil profiles in the field, where the variable z represents the depth in the top profile fully wetted by ponded infiltration.…”