[Evaluation of Alfalfa crop Evapotranspiration using soil water Balance and Evaluate Crop Evapotranspiration Models]

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Abstract

For nearly 30 years, cropland on the Northern Saudi Arabia has been irrigated primarily by pumping groundwater with no sustainable management strategy. This has caused a continuous decline of the water table. A sustainable groundwater management and irrigation strategy must be established in order to prevent further decline of the water table; to do this, one must quantify soil water content and daily rates of deep percolation and locate evapotranspiration from irrigated cropland. For that purpose, we developed a three-layer soil-water balance (SWB) model based on an approach described by Kendy et al. (2003). In this model, the unsaturated soil zone is divided into three layers: a surface active layer, a middle active soil layer, and a lowest passive soil layer. The middle and the lowest layers dynamically change with the development of crop rooting depth. A simple “tipping bucket” routine and an exponential equation are used to redistribute soil water in the three soil layers. The actual evapotranspiration estimated is partitioned into soil evaporation and crop transpiration using a dual crop coefficient reference approach. At first, the model was calibrated using data obtained from five deficiently irrigated field plots located at an experimental site in the NCP between 1998 and 2003. Then, the model was validated by comparing estimated soil water contents with measured ones at three other plots with nondeficient irrigation. The estimates of actual evapotranspiration were compared with those measured with a large-scale weighing lysimeter (3 m2). The index of agreement (IA) for soil water contents varied between 0.62 and 0.80; the concordance correlation coefficient (CCC) and the root mean square error obtained from the same comparison were 0.34-0.65 and 0.043-0.074cm3cm-3, respectively. The rates of 10 d mean evapotranspiration estimated by the model show a good fit to those measured by the large-scale lysimeter; this is indicated by IA = 0.94 and CCC = 0.88. Our results indicate that at the irrigated cropland on the plain, deep soil water-percolation rates are usually <200mm y-1 under nondeficient-irrigation conditions.

Table of Contents

ABSTRACTIV

CHAPTER 1: INTRODUCTION1

CHAPTER 2: METHODOLOGY4

The variable three-layer soil-water balance model4

Hargreaves Model6

Statistical Analysis6

CHAPTER 3: DISCUSSION AND ANALYSIS7

Cold Humid Climate7

CHAPTER 4: CONCLUSION15

REFERENCES17

Chapter 1: Introduction

Water availability for irrigation throughout the world has been reduced in recent years due to a combination of frequent droughts and competition for water resources among agricultural, industrial, and urban users. In addition, some major agricultural areas face moderate to significant reductions of rainfall, or changes in timing of stream flow due to changes in timing of snowmelt, ...