EVAPOTRANSPIRATION AND IRRIGATION WATER REQUIREMENTS EVALUATION OF CHINAROK AREA USING ASCE PENMAN- MONTEITH METHOD

The Koya Directorate of Irrigation (KDI) has a plan to develop the agriculture and launch new agricultural projects in Chinarok area in particular for forestry and orchards plantation. This development requires quantifying the amount of irrigation water and evapotranspiration for the vegetated area. In this paper, these requirements were investigated and evaluated. Chinarok is a rural area located in Kurdistan region north of Iraq. The (KDI) classified the area into three major vegetation types; turfgrass, orchards and forests. Based on the metrological records and plants physical properties, an evapotranspiration (ET) has to be evaluated at the drought summer season, where maximum value is expected. The ET was evaluated for the three vegetation covers by using Penman-Monteith equation which was standardized by the American Society of Civil Engineers and known as ASCEPenman-Monteith equation which is the most reliable method in estimating ET. It was found that ET values evaluated by Penman-Monteith method showed good agreement with experimental results of ET of a published data. Irrigation water requirement in terms of depth and irrigation frequency were evaluated for the three sectors of vegetation based on soil moisture deficit. In addition, irrigation requirements were calculated in terms of volume and daily water demand. The capacity of ground storage reservoir (or storage pond) was recommended as 5400 m 3 to meet daily water demand. These findings provide a base for the design and operation of proposed irrigation systems in Chinarok.


INTRODUCTION
Water enters a plant through its roots then moves upward through the plant to the leaves. A very small amount of water taken up is used for plant growth, and the rest of water transpires out of the plant through stomatal pores. This process is called transpiration .Water can also be lost from the plant site directly by evaporation from plant leaves or soil surfaces (i.e., the intercepted precipitation on the plant foliage). The water needs of a plant thus consist of transpiration and evaporation and are called evapotranspiration, ET, or consumptive use. ET is measured as a depth per unit time such as (in) or (mm) per day, per week, or per month. Knowledge of consumptive use helps in determining irrigation requirement at the farm. ET can be computed by one of the several methods available for the purpose. These methods range in sophistication from simple temperature correlation such as the Blaney-Criddle formula to equations (such as Penman's equation) which account for radiation energy and weather parameters. Most people in Kurdistan region now live in urban and suburban centers where concrete, steel, glass, asphalt, building, and cars prevail; vegetation directly influences these environments in a positive way. Actively growing vegetative surfaces reduce high summer ground surface temperatures due to transpirational cooling. Turfgrass and other landscape plants reduce discomforting glare and noise. Soil erosion, dust, and fire danger are reduced or eliminated on turfed surfaces. For this purpose, Koya Directorate of Irrigation (KDI) tends to create new green areas in Chinarok region. Chinarok is a rural area located to the east of Koya city in Kurdistan region North of Iraq. The source of irrigation water is Hizob stream which flows through a valley some kilometers to the east of Chinarok. The region is characterized as undulated area with steep slope at some locations. The (KDI) suggests the supply of irrigation water by pumping from Hizob stream and to be stored in Chinarok in ground storage reservoir or in storage pond. The aim is to determine the amount of irrigation water to be transferred by pumping from the stream.

Chinarok area
Gross area of Chinarok has been estimated as 200 hectares. The classification of this area is decided by (KDI). The decision which was made is to make rural area forming about 25% (50 hectares) for residence, utilities, and water distribution facilities. The rest which is 75% (150 hectares) is of cultivable areas which have been divided into 80 hectares for orchards and 70 hectares for green areas consisting of 62 hectares for forests areas and 8 hectares for turfgrass (lawn grass). The orchards have to be planted with almond with few cherry and pomegranates, while green areas of forests have to be planted with pine trees of slash species where they are locally planted. Because the region is undulated, the (KDI) suggested the following methods of irrigation: the turfgrass area is to be irrigated by sprinkler, while orchards and forests areas by trickle method. The (KDI) was classified the soil of the region as a sandy loam soil. Chinarok is located at longitude 44°33' (44.55°), latitude 36°5' (36.08°) and altitude 610m. In this paper, an irrigation requirement is to be evaluated at the drought summer season. In the recent years maximum air temperature was recorded in August and the (KDI) provides metrological data for the region for 31 days of August with maximum and minimum temperatures recorded at daytime and nighttime respectively. An average values are considered for the month of August, average maximum temperature = 41.5 C°, average minimum temperature = 27.3 C°, average maximum humidity = 41.4%, average minimum humidity = 11.3%, average wind speed = 2.36 m/s, and rainfall = 0. The temperature, humidity, and wind speed were recorded at 5 m above the ground level in Koya metrological station, KDI, 2017 (13).

Asce penman-monteith method
The American Society of Civil Engineers-Penman-Monteith equation (ASCE-PM) is based on the Penman-Monteith form of the Penman combination equation and is widely accepted as the best-performing method for estimating evapotranspiration (ET) from metrological data, Todorovic, (24). Jensen, Jensen et al. (11) compared 20 methods of computing ET for arid and humid locations. They found that the Penman-Monteith method was the most accurate for either environment. Because of its reliability, the Penman-Monteith method is used when air temperature, relative humidity, wind speed, and solar radiation data are available or can be reliably estimated. The Penman-Monteith method has been recommended as the primary method for defining grass-reference ET o , Allen et al., (3) . The basic hypothesis of the Penman-Monteith approach is that transpiration of water through leaves is composed of three serial processes: the transport of water through the surface of the leaves against a surface or canopy resistance, molecular diffusion against a molecular boundary layer, and turbulent transport against an aerodynamic resistance between the layer in the immediate vicinity of the canopy surface and the planetary boundary layer. The ASCE-PM equation for estimating the crop evapotranspiration, ET C , from vegetative surfaces where availability of water is not a limiting factor is given by Where ρ w is the density of water; λ is the latent heat of vaporization of water; Δ is the gradient of saturated vapor pressure versus temperature curve; R n is the net radiation (solar plus long wave); G is the soil heat flux; ρa is the density of moist air; c p is the specific heat of moist air (= 1.013 kJ/(kg °C); e s is the saturation vapor pressure; e a is the ambient vapor pressure; r a is the aerodynamic resistance to vapor and heat diffusion; γ is the psychromertic constant; and r s is the bulk surface resistance.
Where z m is the height of wind measurement, d is the zero-plane displacement height, z h is the height of air temperature and humidity measurements, z om is the roughness length governing momentum transfer, z oh is the roughness length governing heat and vapor transfers, k is the von Karman constant, and u z is the wind speed measured at height z m . Typically for fully covered uniform crops, d and z om are related to the crop height, h, by, Brutsaert, (5): Since the momentum transfer governs the heat and vapor transfer, the roughness height z oh is assumed to be a function of z om , where For tall and partially covering crops a = 1 and for fully covering crops a = 0.

= (5)
The stomatal resistance of a leaf (rl), is a physiological resistance in (s/m) imposed by the vegetation itself and LAI active is the active (sunlight) leaf-area index (dimensionless). On sunny days, the stomatal resistance on exposed leaves decreases rapidly at sunrise, remains at a minimum value all day if the water supply to the leaf is adequate and increases at sunset. The bulk stomatal resistance, rl, of a single well-illuminated leaf typically has a value in the order of 100 (s/m), and the leaf area index, LAI, is defined as the surface area of the leaves (upper side only) to the projection of the vegetation on the ground surface. The LAI for grass and alfalfa can be estimated using the following relation, Allen et al., For the standard turfgrass with 0.12 m height and stomatal resistance, rl =100 s/m. Equations (6), (7), and (5)  The net radiation, R n , is equal to the net solar (shortwave) radiation, S n , plus the net long wave radiation, L n , David, (8) hence; = + (9) A direct measurements of net radiation is not available in the area and is usually difficult to measure because net radiometers are hard to maintain and calibrate, as a consequence, the net radiation is often predicted using empirical equations. The net short wave radiation can be estimated using the equation, David, (8) Using the radian mode in Equations (11), (12),  (14), yields L n = -3.88 MJ/ m 2 .day, the negative value indicates that the net long wave radiation in August is away from the earth. The total available energy, R n , in turfgrass area in August is then equal to the sum of S n and L n and is given by Eq. (9) as: = + = 15.33 − 3.88 = 11.45 2 .

Soil heat flux, G
The soil heat flux, G, (in MJ/m 2 .d) is the energy utilized in heating the soil, and is positive when the soil is warming and negative when the soil is cooling. Averaged value of G over one day is typically small, but becomes more significant for hourly or monthly time periods. For daily time interval beneath a dense cover of grass surface it can be assumed G day = 0, David, (8).

Psychrometric constant, γ
The psychrometric constant, γ, depends on the atmospheric pressure, P, and the latent heat of vaporization, λ, and is defined as, David, (8):  (19). On the other hand, the aerodynamic resistance, r a , for trees is an order of magnitude less than for grass, because trees are not only taller but also present a relatively rougher surface to the wind and so are more efficient in generating the force eddy convection which in most metrological conditions is the dominant mechanism of vertical water vapor transport, Calder, (6). The main problem lies in the difficulty of obtaining some measurement of the trees vegetation factors, especially r s and r a which is a complex function of many climatological and plant biological factors. This was admitted by Kelliher, Kelliher et al., (15) when they studied a stomatal control at a plant leaf. He found a maximum stomatal conductance, g max (the inverse of stomatal resistance r l ) for main types of vegetation covers, for deciduous trees g max = 4.6 mm/s and for conifers g max = 5.7 mm/s as it is mentioned by Ward (Ward and Robinson, 2000).Thereafter a stomatal resistance r l , is calculated as (r l = 1/ g max ) which represent the minimum resistance. For orchards trees (deciduous), stomatal resistance r l is 21.74×10 -2 s/mm (= 217.4 s/m) and for pine trees (conifers), r l is 17.54×10 -2 s/mm (= 175.4 s/m), then the surface resistance r s for trees is evaluated by Eq. (5). Leaf area index LAI for almonds and pine trees have to be investigated carefully. Many researches were conducted to estimate LAI for trees because it is a key parameter for estimating plantation and forests productivity. Jose and his colleagues, Jose et al., (12). were evaluated LAI in almonds orchards by using hemispherical photography technique also called the fisheye photography, the LAI was obtained in mid-season of almonds orchards and ranging from 1.8 to 2.6 m 2 / m 2 , the mean value of 2.2 m 2 / m 2 can be adopted to determine r s , so Eq. (5) gives r s for orchards trees ≈ 198 s/m (2.3×10 -3 day/m). Carlos and his colleagues were performed an analysis using loblolly and slash pine LAI data for long-term experiment. Mean annual LAI for slash pine was found as 2.5 m 2 / m 2 , Carlos et al., (7) and by Eq. (5), r s for pine trees ≈ 140 s/m (1.62×10 -3 day/m). In 1995, Monteith , Monteith, (18) reached a relationship between r s and r a in trees vegetation and pine forests. He found out the ratio r s /r a was of the order of 10, whereby, this approximation is of most significance in reducing the problem in estimating r a when r s is known. Using Monteith s ratio (r a = r s /10), r a for almonds trees can be estimated as 2.3×10 -4 day/m and r a for pine trees is 1.62×10 -4 day/m. The albedo or canopy reflection coefficient, α, for forests and crops are given in table (1), and average values can be considered, for forests = 0.14 and for orchards (tall crops) = 0.18, then short wave radiation, S n and net radiation, R n for forests and orchards are evaluated by Eq.
(10) and Eq.(9) respectively. Other parameters of ASCE P-M equation; L n , Δ, ρ a , ρ w , λ, γ, and c p remain constant as those for turfgrass, thereafter evapotranspiration ET c , for orchards and forests area are evaluated by Eq. (1). A summary of parameters calculations and results are given in table (2) for the three types of vegetation; turfgrass, orchards, and forests. The results of ASCE-PM equation refer to the average daily evapotranspiration, ET, based on the averaged maximum weather conditions in summer during August and at maximum leaf conductance (at minimum stomatal resistance). ET, for turfgrass area is 6.5 mm/d, for orchards area is 8.5 mm/d, and for forests area is 11.5 mm/d. It is clear that ET for orchards and forests areas is more than that of turfgrass,

Table 2. Summary of Parameters Evaluation and Results of P-M Equation
this is because of aerodynamic resistance for trees (tall plants) being much less than that for grass (short plants) also because of rough surface of trees (less albedo, α) in which the net solar energy absorbed by trees (R n = 13.24 MJ/ m 2 .day) is greater compared with that absorbed by grass vegetation (R n = 11.45 MJ/ m 2 .day).

Validation of the results
Evapotranspiration have been established for most commonly used warm and cool season turfgrass species in United States. In California, both cool and warm season species are grown in major populated area of the state and ET for warm season turfgrass has been measured by crop-coefficient approach and was given in the range 0.24-0.28 in/ day (6.1-7.1 mm/day), Ali et al., (2). In California, the almond board of California had measured and recorded ET for almond trees also by cropcoefficient approach and maximum value of 9.61 in/ month (7.9 mm/d) was recorded in July and 8.59 in/ month (7 mm/d) was recorded in August , Larry, (16). Pine trees ET has been explored for the slash pine species by the trees planted in a weighted lysimeter in Florida State. The results of the lysimeter study showed that seasonal averages were weighted by the lengths of periods and came to 2.4 mm/day for the autumn months (October, November, December), 1.2 mm/day for the winter months (January, February, March),  shown in table 2 showed a good convergence with the measured published data of ET for the same covers.

Net irrigation requirement and irrigation scheduling
Irrigation water requirement refers to the depth of water required to meet evapotranspiration requirement. In other terms, it is the depth of water required to bring soil moisture in root zone of the plant from permanent wilting point to the field capacity. Field capacity (F C ) is the moisture content retained in the soil after excess water being removed by drainage. Permanente wilting point (PWP) is the lower limit of moisture in the soil at which plant cannot extract water essential for its growth. The amount of moisture content between field capacity and wilting point is termed as available moisture (AW). Available Moisture content of soil can also be represented as equivalent depth of water (d) or depth of irrigation, Israelsen et al., (9) and is given as: Where ΔP w is the difference in moisture content (by weight) between field capacity and wetting point (F C -PWP) also called as soil moisture deficit, A s is the apparent specific gravity of soil, d, is a net depth of irrigation, and D, is the effective root zone depth of plant. Soil moisture near wilting point is not readily available moisture to the plant, Hence, the term readily available moisture has been used to refer to that portion of moisture that is most easily extracted by plants. The suggested depletion of available soil water is 75 percent of the total available moisture, also known as moisture depletion percent, P D , Israelsen et al., and net depth of irrigation in Eq. (23) can be modified to: It should be noted that, because of the capacity of soil to store water, it is not necessary to apply water to the soil every day even though the evapotranspiration takes place continuously. Soil moisture can vary between the field capacity and the permanent wilting point. The average moisture content will thus depend on the frequency of irrigation and quantity of water applied. Thus frequency of irrigation (irrigation interval) is calculated by dividing the amount of soil moisture which may be depleted within the root-zone soil by the rate of evapotranspiration, Asawa, (4) irrigation frequency is: Where, I.F, is the irrigation frequency (time period) between two successive irrigations in day. In field irrigation practices, the total depth of water applied to the cropped field should include water lost during water application and other factors contribute to large losses of irrigation water which, in turn, reduce irrigation efficiency. Thus gross irrigation requirement is equal to net irrigation requirement plus losses. The gross depth of irrigation, GDI, is thus expressed in terms of water application efficiency, E a , as: Trickle irrigation systems typically apply small amount of water on a frequent basis, maintaining soil water near field capacity, but usually not the entire soil surface is wetted, the system is applying water to each individual plant using one or more emission points per plant, thus in trickle method some modifications are required in the design. The modifications require determining wet area, wetted pattern, and water movement in the soil. In trickle system, typically trees are planted in rows where each trickle lateral pipe irrigates one row of trees. In this paper spacing between trees is denoted as (S p × S r ) where, S p , is spacing between trees and, S r , is spacing between rows (or between lateral trickle lines). The (KDI) decided (S p × S r ) for orchards trees as (4×4 m), and for forests trees by (6×6 m). Hence each one tree would occupy area of (S p × S r ) m 2 . Figures 1 and 2 show layout and spacing between trees irrigated by trickle system in orchards and forests area. In trickle system, wetted area depends on soil texture and can be estimated from special tables.
Where NDI max is the maximum net depth of irrigation in one irrigation cycle and P W is the percent of wetted area. In trickle irrigation, water is applied directly to the root zone of plants without coverage of entire area of a field, so the evaporation from soil surface and crops leaves is too small, thus the main component of water consumption in trickle irrigation is transpiration. The daily transpiration rate in trickle system is based on daily ET and the percent of area shaded (covered) by plant leaves, Ahmad and Hakqi, (1) as given: is the daily evapotranspiration in (mm/d), P s is the percent of shaded area; T d is the daily transpiration in (mm/d). The maximum percent, P s , for a mature orchard is usually about π/4 (=0.785), which is the ratio of a circle (tree canopy area) enclosed by a square area (area occupied tree), (Keller et al., 1990). Similarly irrigation frequency in trickle irrigation is restricted by the maximum limit as in the formula, Ahmad and Hakqi, (1): And gross irrigation requirement is given as:   (20).  (4). The results refer to, NDI and GDI in orchards is much greater than that in forests. This is due to percent of wetted area, in orchards, P W =44% while in forests, P W = 20%, but irrigation in forests sector is more frequent (3 days interval) than that of orchards. This means; apply small depth of water in forests with more frequent irrigations. The volume of irrigation water in one irrigation cycle for the three sectors is calculated by multiplying GDI by wetted area, A W , and is given in table (4). For the land irrigation scheduling, it is necessary to find daily water demand. The volume of water needed in each irrigation cycle should provide during period equal I.F (number of days between irrigations). In other words, irrigation is needed when the available water that is present in root zone is depleted. In this manner the volume of water needed in one irrigation cycle is divided by I.F to obtain daily volume demand (m 3 /day). Thus using table (4), in turfgrass area, the daily water demand is (720 m 3 /d), in orchards area, is (3129 m 3 /d), and in forests area, is (1490 m 3 /d). The total daily demand is the sum of all which is equal to 5339 m 3 /d and this represents maximum daily demand during hot summer. The volume of ground storage reservoir (or storage pond) is then can be recommended as 5400 m 3 to meet averaged maximum daily water demand that is evaluated at the drought weather conditions of August. In addition, number of trees in orchards and forests area can be determined by dividing the total area by the space occupied by one tree (S P × S r ). In orchards area, the number of trees is (800000 ÷ 16 = 50000 trees) and in forests area is (620000 ÷ 36 = 17222 trees ET c in addition to reference ET o when sufficient weather data and crop physical information are available. The maximum evapotranspiration, maximum irrigation water requirements in terms of depth, volume, and daily water demand are evaluated precisely for Chinarok region for the three vegetation covers. Findings of this study enable (KDI) to design and manage the proposed irrigation project of Chinarok. The management is to apply the correct amount of water at a correct time to optimize water uptake by the roots.
Operating the project according to irrigation scheduling will help to reduce the amount of water lost by surface runoff and deep percolation below the root zone, making the project works at the desired efficiency. Because of water supply system of Chinarok is to be by pumping and storage, evaluation of daily water demand is the base in sizing and designing the pipeline and pumping station on Hezob stream.