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Abstract
Abstrak
Evapotranspirasi adalah unsur utama dalam menghitung kebutuhan air tanaman yang kemudian menjadi dasar dalam penjadualan irigasi. Evapotranspirasi dipengaruhi banyak faktor sehingga pengukurannya secara langsung tidak mudah, karena itu dikembangkan banyak model pendugaan untuk mengatasi hal tersebut. Salah satu model yang direkomendasikan FAO adalah metode Penman-Monteith (P-M). Untuk mengetahui apakah metode ini tepat juga digunakan di Propinsi Lampung perlu dilakukan pengujian dengan membandingkan hasil pengamatan langsung (2006-2008) yang dilakukan di dua stasiun pengamatan di Lampung yaitu Branti dan Masgar. Hasil pengamatan di Branti rata-rata lebih rendah dari hasil metode P-M pada laju ET > 4 mm, dan lebih tinggi untuk laju ET < 4 mm; sedangkan untuk stasiun Masgar menunjukkan laju ET hasil pengamatan selalu lebih tinggi dari pada hasil perhitungan metode P-M. Hasil metode P-M secara rata-rata 1.09 kali lebih tinggi dari pengamatan Branti dan 0.89 kali lebih rendah dari pengamatan Masgar. Koefisien korelasi antara metode pendugaan dan pengamatan langsung rendah (r= 0.3 untuk Branti dan 0.5 untuk Masgar). Ketidak cocokan ini dapat disebabkan pertama karena ketidak cermatan dalam mengukur penurunan muka air pada panci evaporasi yang terlihat dari rendahnya koefisien korelasi evaporasi pengamatan dengan semua unsur iklim yang berkaitan erat dengan evaporasi (suhu dan kelembaban udara, kecepatan angin dan intensitas radiasi; kedua karena CROPWAT menggunakan data lama penyinaran yang dikonversikan secara linier menjadi intansitas radiasi sedangkan dalam pengamatan langsung hubungan antara lama penyinaran dan intensitas radiasi tidak linier.
Kata kunci: irigasi, evapotranspirasi, pendugaan, Penman-Monteith, CROPWAT
Abstract
Evapotranspiration is an important factor in estimating crops water use and then irigation schedule. Direct measurement of evapotranspiration is difficult since it is influenced by many factors. Estimation methods are developed for estimating evapotranspiration rate from meteorological data. One method which is recommended by FAO is Penman-Monteith Method (P-M). To evaluate whether this method could be accurately used in Lampung a comparison had been conducted with evaporation measurement on two climate stations in Lampung, Branti and Masgar with data set from 2006-2008. The result for Branti showed that observation data was lower than P-M for ET > 4 mm and higher for ET <4; while for Masgar evaporation observation always higher than P-M. In general P-M was 1.09 times higher than observation in Branti and 0.89 lower in Masgar. Correlation coefficients between P-M and observation were low (r = 0.3 for Branti and r= 0.5 for Masgar). Two possible reasons for the disagrrement were first, there was an error in measuring water level on the evaporation pan, this showed by the fact that observed evaporation has low coefficient correlation with all meteorological data which have direct impact on evaporation (air temperature and humidity, wind speed and radiation); second, CROPWAT converted shunshine duration to be the radiation intensity with linear approach while field data showed that sunshine duration did not relate linearly with radiation intensity.
Key words: Irrigation, evapotranspiration, estimation, Penmann-Monteith, CROPWAT
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