Drip irrigation requires less water than other irrigation methods. It requires roughly half the water needed by sprinckler or surface irrigation. Small water sources can be utilized more effectively in the drip irrigation system. In the drip irrigation, partial wetting is formed on the soil surface. The evaporation from this wetting small portion cannot be treated one-dimensional. Early in the season when there is little or no plant cover, transpiration become complex because of local advection. One of the basic research needs at the present time is to obtain the water requirement of one crop. To date, however, there have been few research studies on this matter. The experiment was performed on Mass sandy loam filled in microlysimeters placed in a greenhouse. One of which, soybean was planted and the other was kept bare. Net radiation of an isolated crop was measured rotating the net radiometer around the canopy as shown in Fig. 2. Transpiration was calculated by eq. (9). The accuracy of the estimation was checked using the leaf temperature obtained by thermocouples stuck to the leaf abaxial. The results agreed well when the minimum leaf resistance was equated 1.0 s/cm in eq. (5). Observation of the wetted radius was performed during a month. This was investigated by taking the maximum and minimum radius to approximate the shape ellipse. The result were shown in Fig. 8. To estimate the evaporation from the wetted area, diffusion theory on circle plate was adapted. The cumulative use of water from both cropped and bared lysimeters were shown in Fig. 11. Only 11% of water was evaporated from the cropped soil surface compare to the bare plot. Using the theory accomplished, a simulation model for analyzing three-dimensional transfer of water was developed. Root extraction term was given by eq. (25). The result was shown in Fig. 14.
点滴灌漑方式は他の灌漑方式と比較して,少量の灌漑水量で効果を上げ得ると言われている.この方式では土壌面の一部分に給水されるので,蒸発散現象は1次元的計算が適用できず,消費水量の推定手法の確立が解決されるべき問題として残されてきた.Waggoner and Reifsnyder(1961),Ben-Asher et al.(1978),Thorpe(1978)にわずかにこうした問題への取り組みが見られる程度である.以上の研究経過に鑑み,ここでは蒸発散機構に不明瞭な点の多い単体として栽培された作物を取り上げ,消費水量予測のためのシミュレーションモデルの作成を行った.また,ガラス室内のライシメータに栽培した大豆を用いて蒸散量,土壌面蒸発量の実証試験を行った.作物個体の純放射量はFig.2に示す方法で測定し,これを(9)式に代入して逐次試行法で蒸散量を求めた.計算の精度を葉温について比較したところ,Fig.5に示すとおり天候の不安定な午前中を除けば,γ_min=1.0s/cmが実測とよく適合した.土壌面蒸発量の同定は湿潤面が局部であるため,周囲乾燥面からの移流現象が生じその解析は複雑となる.ライシメータを円形平板に模して拡散理論を適用し,(17)式によって水蒸気輸送抵抗を求めた.その妥当性については円筒モデルによる基礎実験を行い,確かめることができた.Fig.8は等量の滴下水によって植栽区,裸地区の土壌面に形成された湿潤面の広がりを1日1回測定し,約1カ月にわたって描いたものである.作物による水の吸収,土壌面に到達する日射量の遮断等のため,植栽区の土壌面蒸発量はFig.11に示すように裸地区の消費水量の約11%に低下した.以上得られた蒸散および土壌面蒸発についての知見を利用して,点滴灌漑下の土壌中の水分輸送についてモデルを作成した.土壌面蒸発計算に必要な土壌面温度は実測によったが,熱輸送方程式の同時解析により更に精微なモデルへの発展も可能である.