WATER HOLDING CAPACITY OF SOILS IN SINGAPORE

Abstract

Soil moisture is an important component of the hydrological cycle. As such, the study of water retention in soils is significant in hydrology. Important parameters such as the maximum and minimum amount of water that the soil can hold and its average moisture content under a given climatic condition are important in water retention studies. These parameters are determined by soil scientists, mostly through laboratory studies, and generally provide the theoretical aspects of water retention in the soil. However, for practical purposes, basic data collected from the field are vitally important. Unfortunately, no attempts have so far been made to provide such information in a systematic manner. The objective of this study is, therefore, to present some simple parameters describing the water holding capacity of Singapore soils under natural conditions. These include the maximum and minimum water content these soils generally hold, their field capacity and their average moisture content. Two methods were adopted in this study. Firstly, the determination of water content through the periodic collection of disturbed and undisturbed soil samples at various locations in Singapore. Secondly, the extraction of relevant soil water parameters from the various studies done in the past on some other aspects of soil water in Singapore. The sites were selected based on their representatives of the types of soil and vegetation found commonly in Singapore. In addition, the various variables that affected soil water retention, for examples, soil types, vegetation and degree of ground disturbance were also taken into consideration. The results showed that generally the soils in Singapore stayed moist most of the time. The average water content in most cases occurred not too far below the field capacity. Due to high permeability, the maximum water content did not last very long. The water content not only varied with time in response to input and output of water, but also varied with depth. The fluctuations in the water content seemed to be greater in the topsoil and lower horizons of the subsoil than in the middle horizons. These results were explained in terms of the various soil characteristics, for example, texture, structure and composition. The results are useful for various applications; perhaps the most useful being the water budgeting analysis. It appears that the soils never arrive at a deficiency state and for most of the time are close to field capacity than wilting point. This would mean that during the relatively wet periods (eg. the northest monsoon season), little input of water would be required before soils reach field capacity. This surplus water will often be available for runoff or subsurface water recharge as well as free (pore) water in pore spaces. Applications of these for flooding and slope stability are also discussed in this exercise.