Estimation of Crop Water Requirement Using CROPWAT Model for Onion and Tomato, A Case Study of Raya Azebo District, Ethiopia

Water is becoming gradually scarce worldwide. Aridity and drought are considered natural causes of scarcity of water on earth. Man-made desertification and water shortages have further aggravated the natural scarcity of water worldwide. Water scarcity has a huge impact on food production and without water people do not have a means of watering their crops and therefore, to provide food for the fast-growing population, farmers need to increase their output from existing cultivated areas to satisfy the food demand of an increasing population in the world especially in Ethiopia. Irrigation systems essential to enhance crop productivity in order to meet future food demand and ensure food security. The study was carried out to estimate the crop water requirement of onion and tomato in Raya valley, Northern Ethiopia. By using the climatic data, cropevapotranspiration (ETc)and reference crop evapotranspiration (ETo) for each crop were determined using CROPWAT 8.0.The study shows that crop water requirement of onion with a growing period of 100 days to maturity requires 366.0 mm depth of water on off season and 338.9 mm on main season respectively. Crop water requirement of tomato with a growing period of 125 days to maturity requires 509.9 mm depth of water on off season and 472.1 mm on main season respectively. These results can be used for planning and for most efficient water use and to optimize production of the onion and tomato in the study district.


Introduction
Water is the basic requirement for agricultural and the economic development of any country.Water is becoming valuable and scarce due to its increasing demand in agriculture and industrial sector.Agriculture being the backbone of population and exploitation of available water resources to meet the agricultural need requires its scientific management.
The values for crop evapotranspiration and crop water requirement are therefore, identical (except opposite sign), crop waterrequirement refers to the amount of water that needs to be supplied(positive sign), while crop evapotranspiration refers to the amountof water that is lost through evapotranspiration (negative sign).
CROPWAT facilitate the estimation of crop evapotranspiration, irrigation schedule and agricultural water requirements with different cropping patterns for irrigation planning and management. Penman-Monteith method (Allen et al. 1998) has been reported to yield consistently more accurate reference evapotranspiration (ETo) estimates across a wide range of climate condition (Jensen et al. 1990; Kashyap and Panda 2001;Irmak et al. 2003).
Different crops have different water use requirements under the same weather condition. Estimation of crop water requirements (ETc) is one of the main components used in irrigation planning, design and operation (Rowshon et al., 2013). (Jensen et al. 1990), provided detailed reviews of the methods commonly used to determine evapotranspiration and estimated crop water requirements.
In actual fact, CROPWAT is a computer program for irrigationplanning and management, developed by FAO (Abdalla et al. 2010). However, the sameprinciple holds for rain-fed crop production as well. Its basic functionincludes the calculation of reference crop evapotranspiration, cropwater requirement and scheme irrigation requirement. Althoughseveral methods exist to determine ETo, the Penman Monteith methodhas been recommended as the appropriate combination method todetermine ETo from climatic data on: temperature, humidity, sunshineand wind speed are required (Allen et al. 1998).
The crop water requirement (ETc) was found to vary not only with the crops its stage and duration, but also with the season as well. The crops differed in water demand as the growing season changed. During initial stage of the crops, the ETc was less and increased during development stage, reached to its maximum values during mid-season and reduced during crop maturation stages. ETc was maximum for winter season and lowest in summer season. The information generated can be used in scheduling irrigation for different crops.
The FAO-Penman-Montieth equation is recommended as the standard method for estimating reference and crop evapotranspiration as well as crop irrigation water requirement through the FAO CROPWAT model. CROPWAT is one of the models extensively used in the field of water management throughout the world. CROPWAT facilitate the estimation of crop evapotranspiration, irrigation schedule and agricultural water requirements with different cropping patterns for irrigation planning and management. Different crops have different water use requirements under the same weather condition. The study was conducted with the objectives of Estimating Crop Water Requirement using CROPWAT model for most market-oriented crops under different cropping calendars.

Methodology 2.1. Description of the study area
Tigray is located in the northern part of Ethiopia with an altitude ranging between 400 to almost 4,000 m above mean sea level. It is located between 12° 15' N and 14° 57' N, and 36° 27' E and 39° 59' E. It covers an area of about 53,000 km² (CSA, 2015). The study was conducted in Raya Azebo district, SouthernZone of Tigray, Ethiopia.Administratively, Raya Azebo is subdivided into 18 kebeles at an altitude ranging from 930 to 2,300 m above mean sea level (Tesfay et al., 2014). The climate is predominantly semi-arid with irregular rainfall accompanied by frequent drought periods.Average annual rainfall ranges from 800 to 1,000 mm per year reducing to 400 mm (Edwards et al., 2006).

Reference evapotranspiration (Eto)
The following table (1) gives the Reference Evapotranspiration (Eto) for all the months in a year, in which the highestreference evapotranspiration is found in the month June (4.7 mm/ day) and the lowest Reference evapotranspiration isfound in the month January (3.3 mm/ day) and November (3.77 mm/ day) due to differential changes occurs in theweather conditions like temperature, sunshine hours, humidity and wind speed.
In dry season, the resulting low relative humidity combined with high temperatures led to increased evapotranspiration over this period of a year. Inversely the low values of Eto in rainy season may be due to the high frequencies of rainfall combined with high relative humidity and relative low temperatures. As the trend of Eto affecting by climatic factors such as temperatures, solar radiation, and rainfall as well as wind, relative humidity of the air consequently Eto is a climatic parameter. With the variations of these parameters Eto were vary greatly within and between seasons. The results are in accordance with Adeniran et al. (Adeniran et al., 2010), which showed that Eto was lowest during the peak of the rainy season to highest during the peak of the dry season.

Crop Water Requirement, Effective rainfall and Irrigation Requirement
From the below results (Table 2), It shows that for onion in off season and main season, it requires 366.0 and 338.9mm crop water requirement, 42.7and 97.6mm effective rain fall and 323.3and 241.3mm irrigation requirement respectively. For tomato in off season and main season, it requires 509.9 and 472.1mm crop water requirement, 60.3 and 122.1mm effective rain fall and 449.6 and350.0mm irrigation requirement respectively. It shows that in off season the crop water requirement of onion and tomato were needed more water than the main season, due to the climatic condition and in the main season the effective rainfall is more available than the off season.

Average monthly maximum, minimum temperature and reference evapotranspiration (ETo)
From the below figure, the Reference Evapotranspiration (ETo) for all the months in a year, in which the highestreference evapotranspiration is found in the month June (4.7mm/ day) and the lowest Reference evapotranspiration isfound in the month January (3.3mm/ day). This result was showed that the crop water requirement in those months require higher amount of water. The maximum temperature was recorded in the month of June followed by may with the value of 35.5 and 32.5 0 c. In the other hand the minimum temperature wasshowed that in the month of December, January and November respectively. y Figure1: Bar Chart indicating long term average monthly minimum, maximum and reference crop evapotranspirationdata of the district

Crop Water Requirement, Effective Rainfall and Irrigation Requirement of onion and tomato
From the below bar graph, onion and tomato were required higher crop water requirement in the off season than in the main season. This reason, due to maximum temperature, higher sunshine hour, high wind speed and reference evapotranspiration (ETo). p p ( )

Conclusion
Crop water requirement of onion with a growing period of 100 days to maturity requires 366.0 mm depth of water on off season and 338.9mm on main season respectively. Crop water requirement of tomato with a growing period of 125 days to maturity requires 509.9 mm depth of water on off season and 472.1mm on main season respectively.From the present study, it is concluded that Reference Crop Evapotranspiration, Effective Rainfall, Crop water requirement and Irrigation water requirement can be estimated using CROPWAT 8.0 Software with the input of climatic data like maximum and minimum temperature, relative humidity, wind speed and sunshine hours and rainfall.It showed the significance of requirement of scientific planning for irrigation. Results on ETc and IR provided practical assessment for irrigation scheduling of onion and tomato grown in these t semi-arid environments. These results can be used for a most efficient water use and to optimize production of the onion and tomato in the study district.