**1. Introduction**

Water and sun radiations are the most important environmental factors that make life possible on earth [1]. The lack of fresh water is one of the greatest concerns of humankind [2]. The rivers, lakes, reservoirs, creaks, or streams are natural water resources important for every living organism [3]. Without water, the earth would be a dead desert [4]. Water is a prerequisite for life, is involved in almost all processes of life on our planet, and has many functions in the climate system as well [2]. All organisms contain 50–90% water, some aquatic organisms even 99% [3]. If water becomes scarce or has poor quality, plants and animals die. Humans must drink about 2 L of water per day [5]. Any other substance cannot substitute the function of water. Around 97% of water on the earth is salty and unsuitable for drinking and irrigation, whereas 1.8% was frozen in glaciers and snow [6]. About 20% of the world's population already is suffering from water scarcity [3]. Water will be the most important substance during this century, and therefore, we need a global water policy guided by the United Nations [7]. These surface water resources are extremely important on daily basis for human population too. The earth's surface is 71% covered by the oceans regarding the total water sources of 97% [3]. The freshwater resources make up only 2.5% of total water [6]. In addition, 75% of freshwater is made up of glaciers and polar ice that leaves less than 1% of available fresh water in liquid form [8]. Soil water has many roles but some of the most important are water as a solvent, temperature buffer, and metabolite activator [9]. All of these roles are incorporated trough the water-plant-soil relations. During the cultivation of certain plants, many of the farmers found that a number of agronomic measures do not give a good result if the irrigation of crops was omitted or neglected. Water deficit was more often associated with nutrient deficiencies through their reduced solubility and limited distribution to the root system rather than reduction of insufficient nutrient amount. Water needs are increasing on a daily basis that is to be expected compared to population growth; however, water resources are declining and could soon become scarce. Of the total water sources, the most demanding is agriculture and the water used for irrigation for a 68%,

while about 21% used for public supply and about 11% accounted for by industry process. Water use around the world has increased six-fold in the past 100 years, twice as fast as the human population, and is expected to double again before 2030, driven mainly by agriculture and irrigation [1]. Water is an important component in every plant's life helping them to obtain their nutrients (through the process of photosynthesis), growth (cell division, mitosis), respiration (cellular respiration), and turgidity (up standing form). Water helps plants to maintain their formation by transporting water and dissolved nutrients, amino acids, and other osmotic active substances from soil to aboveground plant part. Water help plants to perform the most important process, for them, the photosynthesis. In this chapter, we focused in water cycle and its importance for soil to plant life. The soil consists of different horizons, different thicknesses formed under the influence of pedogenetic factors, and processes that have been going on for millions of years and that are constantly going on. Soil system is consisting of solid (soil particles), liquid (water), and gaseous phases (air). A porous space was built between the soil particles of different shape and dimensions in which there is water, air, or some other gas. Vegetable production largely depends on the quality, type, and types of soil, and the necessary factor for plants is water. It is therefore not surprising that all civilizations in the development of human society have settled in the river valleys. Water is constantly present in the soil or on its surface. Its content in the soil is constant changing and depends on weather conditions and the needs of the plant world. The water is in constant circling, and this movement was called the hydrological cycle. Due to the movement of water in the soil and variable content, there are two major problems. One of those problems is excess water in the soil, so due to such water-air regime, unfavorable living conditions for plants occur. Another problem was the lack of water in the soil for normal growth and development of plants that was negatively reflected on yield. The fact is that producers were increasingly faced with prolonged droughts during the growing season. The only measures to combat the consequences of such troubles are the introduction of programs of the irrigation through reclamation measures as a necessary item for the future of agricultural production and reduction of far-reaching consequences if not prepare for the changes that have taken place. The biggest challenge of today's agriculture is how to ensure sufficient water needs for growing crop. This entails to identify critical plant growth phase in order to meet their water needs. Defining the period, form, role, and amount of available moisture for plants could be useful for obtaining the optimal yields.
