A mathematical model to simulate evapotranspiration was developed and used to calculate the evapotranspiration from a small soybean field. The calculated evapotranspiration rates were compared with the estimation obtained by the energy balance method. Observations were carried out on sunny days from late July to August in 1976. The analysis of the data at the 12th August is presented in this paper. When the energy balance method is applied to a small field, advective energy from surroundings must be considered. In such cases, the equation to estimate the evapotranspiration rates should be modified as ET_0 =[(S-G)+ρ∫^H_H_0 u(Bλ(∂q/∂x) – c_p (∂T/∂x) dz ] /(1+B) (1) where S is the net radiation, G is the soil heat flux, p is the density of air, u is the horizontal wind speed, B is the Bowen ratio at some height H, A is the latent heat of vaporization of water, q is the specific humidity, c,, is the specific heat of air, T is the temperature, Ho is the crop height and x is the fetch. Plentiful water was irrigated before measurements to get rid of the stress on water uptake by root system due to soil water deficit. Hourly evapotranspiration rates were calculated with the equation (1) and compared with the results obtained from the conventional energy balance method. The contribution of advection to the evaportanspiration was estimated to be 5 % at its maximum. The value was rather smaller than that of expected because surroundings were vegetated fields such as sweet potatoes, corns, clovers and so on. Applicability of the proposed model for estimating evapotranspiration was examined. In this canopy model, following equations were included: S(ζ) = S_exp [-α∫^ζ_0 F(ζ)dζ] (2) K(ζ) = K_exp [-β(ζ/H_0)] (3) and r_s = r_min
C) (4) whereζis the depth measured from the top of the crop, S (ζ) is the net radiation at the depthζ, F (ζ) is the leaf area density, K and K (ζ) are exchange coefficients at the top and the depthζrespectively, S_leaf is the absorbed net radiation per leaf area, C is a constant and r_s is the resistance of stomata for transport of vapour. The effects of three parameters α, β and r_min on the evapotranspiration rates, which are considered to be characteristic to each canopy, are evaluated. The parameter r_min was found to be most influencial. The hourly variation of evapotranspiration rates were simulated for three values of r_min, 0. 5, 0. 7 and 1. 0 sec cm^<-1>. The evapotranspiration rates thus calculated agreed closely with the ones estimated by energy balance method, especially when the value of r_min was 0. 5 sec cm^<-1>. In this paper, considerations on the adjustment of stomata was not fully made. On this point the researches like as developed by Penning de Vries (1972) must be taken into account for further development. Adding simulation models of soil water movement and heat flow to this canopy model may help to advance systematic understanding of water movement on the caltivated land.
大豆群落からの蒸発散量を熱収支法およびシミュレーションモデルを用いて求めた.測定は畦間が大豆群落によつてほぼ覆われた7月下旬から8月にかけて行ない,ここではとくに8月12日の解析例を示した.熱収支法の適用は小区画(13×13m)の圃場であつたため,熱および水蒸気の移流補正を考慮する必要があると思われた.とくに土壌水分欠乏による吸水過程へのストレスを除くため,測定前に十分な灌水を行なつたので地表面熱収支構造の相違によつて周辺からの移流の観測が期待された.潜熱フラックスの計算に(10)式を用いた結果,正午頃で最も大きく+5%の補正が必要であつた.概して小さな値が得られたのは周囲が甘庶,牧草等の緑地であつたためと思われる.シミュレーションモデルの蒸発散量推定法としての適用性について若干の検討を行なつた.入力データの内とくに(33)式の群落内純放射減衰係数(α),(35)式の拡散係数減衰係数(β),および(38)式の最小気孔抵抗(r_min)の定量化しにくい3つのパラメーターについて蒸発散量におよぼす影響を調べた.その結果r_minの影響が他に比べて大きいことがわかつた.そこでα,β の値を妥当な値で固定しr_minを0.5,0.7,1.0 sec cm^<-1>の3通りの変化をさせて日蒸発散量の数値計算を行なつた.熱収支法との比較では0.5 sec cm^<-1>がよい一致を示した.今回は葉の気孔開閉運動に関する十分な考察は行なわなかつたが,Penning de Vries (1972)の研究をモデルに取り入れることが今後の課題である。さらに土壌中の水分や熱の移動モデルと組み合わせることにより耕地系としての水分の動態の把握が可能となるであろう.本研究を行なうにあたり,観測圃場や測器の使用等に多大の便宜を与えられた九州大学農学部栽培学教室ならびに農業気象学教室諸氏に謹しんで感謝の意を表する.また論文作成にあたつて懇篤なる御校閲をいただいた灌漑利水工学教室長教授,黒田助教授ならびに京都大学農学部作物学教室高見晋一氏に厚く御礼申し上げる.なお本研究は1976年度の科学研究費の補助を受けて行なつたものである,