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TL 2130 Hidrologi

TL 2130 Hidrologi. Infiltrasi ( Infiltration ). q. Saturation Zone. Transition Zone. Transmission Zone. Wetting Zone. Wetting Front. depth. Infiltration. General Process of water penetrating from ground into soil Factors affecting

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TL 2130 Hidrologi

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  1. TL 2130 Hidrologi Infiltrasi (Infiltration)

  2. q Saturation Zone Transition Zone Transmission Zone Wetting Zone Wetting Front depth Infiltration • General • Process of water penetrating from ground into soil • Factors affecting • Condition of soil surface, vegetative cover, soil properties, hydraulic conductivity, antecedent soil moisture • Four zones • Saturated, transmission, wetting, and wetting front

  3. Definitions Element of soil, V (Saturated) Pore with water solid Pore with air Element of soil, V (Unsaturated)

  4. Lapisan Tak Jenuh (Unsaturated zone)= lapisan yang tidak seluruh pori-pori mikro tanah terisi air • Lapisan jenuh air (saturated zone) = lap yang seluruh pori-pori tanah terendam air  Gaya gravitasi dan gaya kapiler • Porositas (porosity)= prosentase bagian suatu material yang berupa pori-pori terhadap volume totalnya • Porositas total= vol. pori—pori dalam satu material (berhubungan dan terisolir) vol. keseluruhan dari material • Porositas efektif= vol. pori-pori bersambungan atau berhubungan volume keseluruhan

  5. Kapasitas lapang (field capacity/specific retention)= besarnya kandungan air maximum yang dapat ditahan oleh tanah terhadap gaya tarik gravitasi - kerikil : 1 – 10% - pasir : 5 – 20% - clay : 25 - 50% Tumbuhan mengisap air dari sumber ini. • Koefisien layu/kelembaban kritik (wilting point)= kandungan air pada tanah, dengan tumbuh-tumbuhan dalam keadaan layu permanen dan akan mati bila air tidak ditambahkan • Abstraksi awal (initial abstraction)= jumlah intersepsi dan penampungan cekungan yang harus dipenuhi lebih dahulu, sebelum terjadinya limpasan hujan (overland flow)

  6. Soil moisture content= nilai prosentase kebasahan/lembab tanah SMC = kehilangan berat pada waktu tanah mengering berat tanah dalam keadaan basah SMC = berat tanah dalam keadaan basah – berat tanah kering berat tanah dalam keadaan basah • Available soil moisture (lembab tanah yang tersedia)= selisih antara nilai kebasahan tanah dalam keadaan kapasitas lapangan dan kebasahan tanah dalam keadaan titik layu permanen. Hal ini menggambarkan kebasahan yang dapat disimpan dalam tanah untuk menunjang kelanjutan hidup dari tanaman.

  7. Infiltrasi vs perkolasi • Infiltrasi= proses meresapnya air/proses pelaluan air, ke dalam tanah melalui permukaan tanah. Kebalikan: mata air, perembesan (seepage) • Perkolasi= pergerakan air di dalam tanah melalui soil moisture zone (lingkungan sejumlah kecil air diantara sela-sela tanah yang menyebabkan kebasahan tanah) pada unsaturated zone, sampai mencapai muka air tanah pada saturated zone

  8. Kapasitas infiltrasi (infiltration capacity= fp)= kec. infiltrasi maks. yang bisa terjadi kap. pada awal hujan besar  mengecil bila profil tanah sudah jenuh (1-2 jam hujan). • Kecepatan infiltrasi (infiltration rate= fa)= kec. infiltrasi yang terjadi sesungguhnya) Dipengaruhi oleh intensitas hujan dan kapasitas infiltrasi fa<fp bila i < fp fa=fp bila i ≥ fp

  9. Infiltrasi • Infiltration rate • Rate at which water enters the soil at the surface (in/hr or cm/hr) • Cumulative infiltration • Accumulated depth of water infiltrating during given time period

  10. Kapasitas perkolasi (percolation capacity=Pp)= kecepatan perkolasi maksimum Jumlah perkolasi (mm)= jumlah infiltrasi yang terjadi (mm) – jumlah air yang diperlukan untuk pengisian kelembaban tanah (soil moisture) (mm) • Kecepatan perkolasi (percolation rate=Pa)= kecepatan perkolasi yang sesungguhnya terjadi. Nilai ini dipengaruhi oleh kecepatan infiltrasi dan kapasitas perkolasi.

  11. Faktor-faktor yang mempengaruhi kapasitas infiltrasi • Karakteristik hujan: lama hujan, intensitas hujan terjadi pengurangan kapasitas infiltrasi secara konstan, karena: - pemadatan permukaan tanah - pembengkakan tanah liat dan humus - penyumbatan pori-pori oleh partikel kecil - terjeratnya gelembung-gelembung udara

  12. Kondisi permukaan tanah; ada/tidak ada tanaman • Karakteristik tanah; tekstur dan struktur Tekstur= aspek geometris dari partikel komponen suatu batuan, termasuk ukuran, bentuk dan aturan susunan butir-butir tanah Struktur= perbedaan-perbedaan dari bidang perlapisan yang normal (paralel atau sejajar)  proses sedimentasi, tektonik • Pengerjaan tanah • Kondisi klimatologi • Nilai lembab tanah (soil moisture) • Kedalaman air yang tertahan di permukaan tanah (D) serta ketebalan lapisan tak jenuh (L)

  13. Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system. Soil Particle Sizes

  14. Soil Texture Triangle Source: USDA Soil Survey Manual Chapter 3

  15. Soil Water Content Soil Water Content

  16. Infiltration Methods • Horton and Phillips • Infiltration models developed as approximate solutions of an exact theory (Richard’s Equation) • Green – Ampt • Infiltration model developed from an approximate theory to an exact solution

  17. Hortonian Infiltration • Recall Richard’s Equation • Assume K and D are constants, not a function of q or z • Solve for moisture diffusion at surface f0 initial infiltration rate, fc is constant rate and k is decay constant

  18. Hortonian Infiltration

  19. Philips Equation • Recall Richard’s Equation • Assume K and D are functions of q, not z • Solution • Two terms represent effects of • Suction head • Gravity head • S – Sorptivity • Function of soil suction potential • Found from experiment

  20. Green – Ampt Infiltration Ponded Water Ground Surface Wetted Zone Wetting Front Dry Soil

  21. Green – Ampt Infiltration (Cont.) Ground Surface Wetted Zone • Apply finite difference to the derivative, between • Ground surface • Wetting front Wetting Front Dry Soil

  22. Green – Ampt Infiltration (Cont.) Ground Surface Wetted Zone Wetting Front Dry Soil Evaluate the constant of integration Integrate

  23. Green – Ampt Infiltration (Cont.) Ground Surface Wetted Zone Wetting Front Dry Soil Nonlinear equation, requiring iterative solution. • ψ is wetting front soil suction head; • θ is porosity; • K is Hydraulic conductivity; • F is the total volume already infiltrated

  24. Soil Parameters • Green-Ampt model requires • Hydraulic conductivity, Porosity, Wetting Front Suction Head • Brooks and Corey Effective saturation Effective porosity

  25. Ponding time • Elapsed time between the time rainfall begins and the time water begins to pond on the soil surface (tp)

  26. Potential Infiltration Infiltration rate, f Rainfall Actual Infiltration Time Accumulated Rainfall Infiltration Cumulative Infiltration, F Time Ponding Time • Up to the time of ponding, all rainfall has infiltrated (i = rainfall rate)

  27. Example • Silty-Loam soil, 30% effective saturation, rainfall 5 cm/hr intensity

  28. Infiltrometer Single ring infiltrometer Double ring infiltrometer

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