Theoretical Investigation for a Pipe Drainage Design

Design of pipe draimage for multipurpose paddy fields (I)

Abstract

Many of the paddy fields in Japan, constituting about 20% the total paddy field area, have recently been converted temporarily to dry farming without major change in the geophysical features of the land or irrigation equipment.
The reason for this is to cut down on the over-production of rice. These paddy fields are referred to as “multipurpose paddy fields” in this paper. They require many miles of additional pipe drains.
Since the publication of the first Standard Design Book of Irrigation and Drainage Engineering, Division of Planning of Pipe Drain in 1955 and its revision in 1977, the development of pipe materials, machines and research on planning and practice have been continuosly carried out.
The authors studied on-farm water management and on the basis of the results obtained, propose a method for the design of pipe drain systems in multipurpose paddy fields.
The following results were obtained;
(1) By equation L=2H√k/D, determination is made of pipe drain spacing (L), where D is the requirement for pipe drains, k, hydraulic conductivity, and H, the depth of the permeable layer (surface cultivated soil layer).
(2) Using equation 1/L²=1/a²+1/b², the spacing of pipe and mole drains for a combined drain system is determined, where a and b are the spacings of pipe and mole drains respectively.
(3) k in this equation must use the modified k_s determined by α(k=αk_s) where α is the modification coefficient and k_s, the hydraulic conductivity observed in a field.
(4) On-farm water management and integrated drainage systems are discussed.