Pengaruh pengambilan air tanah untuk irigasi terhadap keadaan air tanah, tanah dan pendapatan petani

Abstract

Lahan sawah tadah hujan umumnya masih mempunyai tingkat produktivitas dan intensitas tanam yang rendah, akan tetapi potensi sumberdaya lahan dan peluang untuk peningkatan produksi cukup besar. Peningkatan produktivitas, intensitas dan diversitas tanaman pada lahan sawah tadah hujan telah dilaksanakan melalui sistem irigasi air tanah, namun hasilnya belum optimal, di lain pihak pengambilan air tanah diperkirakan menimbulkan dampak merugikan pada lingkung_an, dan eksternalitas pada masyarakat. Tujuan penelitian adalah ; 1) Mengkaji pengaruh pengambilan air tanah untuk irigasi terhadap perubahan kualitas dan kuantitas sumberdaya air tanah, serta perubahan kualitas tanah, 2) Mengkaji pengaruh pengembangan irigasi air tanah terhadap peningkatan produksi pertanian dan pendapatn petani, dan 3) Mengevaluasi pelaksanaan pengelolaan irigasi air tanah, serta membuat rekomendasi pengelolaan irigasi air tanah yang optimum. Penelitian dilaksanakan di Kecamatan Campaka, Kabupaten Purwakarta, Propinsi Jawa Barat. Penelitian dilakukan terhadap lima dari 16 sumur air tanah di Campaka yaitu TW-46 (Campaka Sari), TW-52 (Cibatu), TW-110 (Benteng), TW-79 dan SDPW-03 (Cijunti). Pemompaan air tanah menyebabkan perubahan kualitas dan kuantitas aliran air tanah dalam akuifer, hal ini juga mempengaruhi kepada kualitas air tanah. Hasil penelitian terhadap kualitas air tanah adalah sebagai berikut; 1) Semakin lama uml!.r sumur air tanah, kemasaman meningkat, saat awal pH berkisar antara 7,20 sampai 8,10, dan pada saat penelitian menjadi antara 6,20 sampai 7,20. 2) Kandungan kalium dan klorida cenderung menurun, saat awal kandungan kalium dan klorida tinggi, kemudian terjadi penurunan menjadi 0,041 mg/I untuk kalium dan antara 1,5 sampai 2,5 mg/I untuk klorida. 3) Terjadi peningkatan kandungan magnesium, natrium dan besi yang berasal dari pelarutan batuan di dalam akuifer dan aliran air tanah. 4) Perbedaan lokasi sumur air tanah tidak menyebabkan perbedaan kualitas air tanah, karena berada pada satu akuifer atau akuifer yang saling berhubungan, dan tidak ada pengaruh pengembangan irigasi air tanah terhadap kualitas air sumur penduduk. Perubahan k_uantitas air tanah terjadi apabila pengambilan air tanah dari akuifer melebihi kapasitas pengimbuhannya, disamping terjadinya perubahan tataguna lahan di daerah resapan air tanah. Dengan asumsi pengambilan air tanah setiap hari selama 10 jam, dan kapasitas pompa air tanah adalah 10 1/det untuk sumur menengah dan 20 1/det untuk sumur dalam, ternyata dengan pemompaan lebih dari 80 hari per tahun telah berada diatas kemampuan pengimbuhan.

resources and farm technology availability. Various efforts to increase crop productivity, cropping intensity and crops diversity have already been done in some rainfed area; one of them is the development of ground water irrigation system. Since the most of people in rainfed area are poor farmers, and their income is mostly generated from agricultural activity, the development of ground water irrigation in this area plays an important role for the acceleration of agricultural development and poverty alleviation. However, an over exploitation of ground water for irrigation purposes might have risen some negative impacts to the environment and externalities to the people life. This research aims to observe the impact of ground water development for irrigation purposes in Campaka Sub-district, Purwakarta District of West Java Province. Results of the research conclude that ground water irrigation development rise some effects as follows; 1) increasing of water acidity, magnesium, sodium and iron contents, decreasing of chloride and potassium. 2) Most of pumping capacity are higher than well recharge capability (if pumping more than 80 days a year and 10 hour pumping a day), but without any disturbance on aquifer recharging, this pumping capacity is still possible to increase by decreasing of pumping water level (as consequence the radius of influence will increase) or by decreasing of pumping duration everyday to become less than JO hour/day. 3) Water level in shallow wells have been slightly influenced by ground water exploitation, especially during dry season (15 % respondent influenced), and water level in deep aquifer have also slightly decreased (0.816 m per year in average), and 4) Soil potassium content tends to decrease while soil sodium content tend to increase. Ground water irrigation development in this area has led to some positive impacts on crops productivity which lead to improvement of farmers social economic condition, such as follows; 1) Increasing of farmer net agricultural income for about Rp. 278 668 per year, and total income to about Rp. 582 264 per year. 2) Increasing of crop productivity to be 4.984 ton/ha in wet season and 4.566 ton/ha in first dry season (compared to 2.5 ton/ha in 1991). 3) Increasing of cropping intensity and diversity. Based on the result survey of willingness to pay (WTP), indicate that by improvement of ground water irrigation service and management, farmers would have no objection if the irrigation service fee is increased from Rp. 3500.00/hour (the current payment) to be Rp. 6. 700, 00/hour for intermediate tube well (IITW) and Rp. 7. 650, 00/lwur for deep tube well (DTW). Referring to grounr! water irrigation investment and operational cost, eventhough the irrigation service fee can be increased, but actually the subsidize is still required about Rp. 3700,00/hour for IITW and Rp. 6850,00/hour for DTW.