Chương I NHỮNG QUY ĐỊNH CHUNG
UNIT 17. UNIT 17. METHODS AND STRUCTURES
There are four methods of applying water:
1. By flood, thus wetting all the land surface;
2. By fuưows, thus wetting only part of the ground surface;
3. By sprinkling, in which the soil is wetted with spray;
4. By sub-irrigation, in which the soil is wetted only a little, but in which the subsoil is saturated.
The first three methods come under the general heading of surface irrigation.
Flood irrigation generally requires large streams or canals, gentJe topography (ground slopes should usually be no greater than three percent) and careful levelling of the land. In theory, it should be possible to ensure that every part of the area to be irrigated absorbs the predetermined amount of water but in practice, although some parts usually receive too much. For this reason, flood irrigation is more suited to close-growing crops like rice.
Furrow irrigation is a method by which water runs in furrows, normally made by cultivating between crop rows. The earth is thrown up into ridges between the furrows and the seed are planted in the centre of the ridges. Furrow irrigation is very common because it is adaptable to a great variety of land slopes and soil textures and can be used with either large or small streams of irrigation water.
Structures on irrigation projects include dams, canals, spillwavs.
desilting works, intakes, pumping stations, tunnels pipelines, inverted siphons, chutes, drop, checks, turnouts, measuring devices, wastewavs.
culverts, overchutes, drainage systems, and other features peculiar to the project.
Dams vary from low diversion weirs to high storage structures Distribution channels vary from small farm ditches where the flow can be 62
controlled by movable canvas barriers, to huge supply canals carrying several thousand cfs.
The M arala Ravi canal in W est Pakistan has a maximum capacity of 22,000 cfs. In United States, some sections of the All-American Canal, built to c an y water to Imperial Valley, Southern California, have a bottom width o f 160 ft, a permissible flow depth of 22 ft, and a capacity of 15,000 cfs. Other irrigation structures vary almost as widely.
Hydraulic features of reservoirs, dams, spillways, tunnels, canals, and other m ajor structures are-describled in other sections.
TERMINOLOGIES - Flood irrigation - Furrow irrigation - Sprinkling irrigation - Subirrigation - Inverted siphon - Measuring devices - ft (foot-feet)
- cfs (cubic foot per second) QUESTIONS
1) How many methods are there in the supply of water for iưigation ? 2) W hich structures are necessary for irrigation systems ?
3) How can the flow be controlled on the distribution channel ? U N IT 18. TECHNOLOGY OF THE COASTAL LAND
R E C L A M A T IO N
Coastal land reclamation is a traditional form of agricultural development in our country. The sea-dyke embankment and land expansion toward the sea are processes happening always at the estuary regions and forming newly-reclaimed areas, which have good conditions
: Tưới ngập : Tưới dải : Tưới phun mưa : Tưới nhỏ : Xi phông ngược : Công trình đo nước : Đơn vị đo bằng 0,3048 mét : (Đơn vị đo)
for economic development (agriculture, aquacultutre and breeding of sea-products).
The basic characteristics related to the agriculture exploitation of newly-land region may be generally described as follows :
- Land elevation is low and effected strongly by sea water and saline ground water.
- Soil is salted seriously and drainability of soil is very weak.
- Insufficiency of fresh water, especially in winter crop.
In such complicated natural conditions it must be a general measure of land reclamation with high efficiency including adapt ability based on the water engineering measure for objective of long-term reclamation with combination of partial reclamation and exploitation.
History of coastal land reclamation, expansion of cultivated land in the North delta regions and corresponding studies have confirmed a reasonable technology as follows:
Six-step technology of exploitation guarantees the optimal transformation from newly-reclaimed region to stable agricultural production region.
- First step : Isolating the effect of external salinity by a system of sea dykes and drainage canals (with depth over 1.5m); isolating partly effect of external saline ground water to exploitable region. The saline ground water in the region is drained to a definite place. This is first step and nol ignorable.
- Second step : "Dilution".
Dilution is reclamation measure at the first stage of the saline-soil region. Rain and tide water are used for active leaching in wet season and for planting trees on the saline soil in dry season. If surface-water drainage is improved, the "Dilution" will last from 1 to 3 years, then the diluted land can be cultivated as a rice field with low productivity. In this step, the soil is still ploughing but not cultivating. Rain water and fresh tide are used to dilute salinity in soil and drain out when falling tide Rush and sedge are often planted for saline absorption.
- Third step : Agricultural exploitation at low level (planting autumn paddy rice) is combined with leaching and land reclamation. If the water drainage is carried out intensively and fresh tide is more supplied, this step often lasts about 3 years and then.
- Fourth step can be implemented: Planting autumn paddy rice with stable, high productivity and summer paddy rice with unsteady, low productivity.
- A fter implementing fourth step, the soil is gradually diluted and productivity of summer paddy crop will be increased and stabilized.
- Finally, three c r o p s per year will be cultivated with high, stable productivity (step 6).
The process of expansion to the sea and land reclamation according to above-mentioned technology may be last from 10 to 20 years.
W ith water engineering investment (fresh water supply in summer-autumn crops and leaching water is actively drainaged in autumn crop), the better to manage and exploit water engineering system, the shorter process of reclamation will be.
In sum, basic technical measure for reclamation of newly-expanded land is rice planting with intensive water drainage at the first stage in order to leach saline soil.
The highly-adaptable exploitation of coastal .land can be displayed as the reasonable selection of plants, taking advantage of tide and economic factors when combining the long-term surface leaching and the limited seepage leaching in the natural condition of North coastal regions.
TERMINOLOGIES - Land reclamation - Newly-reclaimed area - Breeding of sea products - Dilution
- Rush, sedge
- Expansion toward the sea (encroachment toward the sea)
: Cải tạo đất
: Vùng đất mới cải tạo : Nuôi trồng hải sản : "Dã mặn", sự pha loãng : Cói, lau lách
: Lấn biển
1) What are the characteristics of a newly-reclaimed area in the process of agriculture exploitation?
2) How many steps are there in the technology of coastal land reclamation?
3) What is "dilution" process? and how long will it last?
4) Which plants are often planted for the purpose of saline absorption?
5) How many crops can be cultivated on the reclaimed land afct completion of the reclamation process?