STATISTICS ASSIGNMENT

Statistics Assignment

Statistics Assignment

Introduction

The prediction of soil water movement is a central feature of agro-hydrological models. However, the treatment of soil water dynamics in many of these models is often approximate as they do not rely on basic flow theory. Many adopt the cascade approaches for hydrological simulations. According to the reviews by Cannavo et al. (2008) and Ranatunga et al. (2008), a large proportion of crop nitrogen models (7 out of 16) and soil water models (13 out of 21) that have been widely applied in Australia employ the cascade approaches for soil water dynamics. Such models also include the newly developed AquaCrop model for irrigation scheduling developed by the FAO. Although the cascade approaches are simple and easy to implement, they do not satisfactorily simulate soil water movement at daily intervals, and so are less accurate in estimating evaporation and water uptake by crops.

Over the last few decades not only has the basic theory of water movement in soil, i.e. the Richards' equation, become generally accepted, but the modeling of soil water dynamics has progressed significantly through advances in mathematics and computer science. The numerical schemes such as the finite element method used for the solution to the basic equation are well developed, and software such as HYDRUS is readily available for 1D or multi-dimensional simulations. In the simulations of soil water dynamics in the soil-crop system, the models using the basic equation such as the SWAP model developed by Kroes et al. (2008) have also developed. A new simple and explicit algorithm for the basic equation has recently been proposed (Yang et al., 2009). Despite the progress made, the take-up of such theory based flow models for the practical uses is still low, largely because of difficulties in making satisfactory estimates of soil hydraulic properties at a field scale. It is, therefore, important to devise reliable methods for estimating soil hydraulic properties for use in such flow theory based models. A major obstacle to the wider application of water simulation models is the lack of easily accessible and representative soil hydraulic properties. To overcome this apparent lack of data, a project was initiated to bring together the available hydraulic data on soils, residing within different institutions in Europe, into one central database. This information has been used to derive a set of pedotransfer functions that can provide a satisfactory alternative to costly and time-consuming direct measurements.

A total of 20 institutions from 12 European countries collaborated in establishing the database of HYdraulic PRoperties of European Soils (HYPRES). As a consequence, it was necessary to standardise both the particle-size and the hydraulic data. Standardization of hydraulic data was achieved by fitting the Mualem-van Genuchten model parameters to the individual *(h) and K(h) hydraulic properties stored in HYPRES.

The HYPRES database contains information on a total of 5521 soil horizons. Each soil horizon was allocated to one of 11 possible soil textural/pedological classes derived from the 6 FAO texture classes (5 mineral and 1 organic) and the two ...