| Abstract | Bankless Channel Irrigation Systems (BCISs) are a surface irrigation system composed of adjacent, terraced bays with an interconnecting channel constructed such that the rim
of the channel is level with the floor of each adjoining bay. The mode of irrigation is similar to Drain Back Level Basins (DBLBs) where the accumulated surface storage of each
upstream bay is used to augment flow to a downstream bay. The systems of this study have been adapted from rice-based layouts to incorporate furrows for row-cropping. It is
this style of BCIS that has generated considerable interest in Australia, particularly in the south east, where the system is used to grow a variety of crops and offers considerable labour and machine efficiency savings. Two defining features of BCISs are a positive field slope which rises from the bankless channel, and the hydraulic interaction between adjoining bays during the recession phase of the upstream bay and the advance phase in the downstream bay. These two features make evaluation challenging and mean no available hydraulic simulation model can simulate irrigation in these systems across an
entire field.
To improve the irrigation performance of BCISs a method of evaluating current performance was required. Consequently, the objectives of this research were to firstly identify appropriate evaluation methods for evaluating BCISs, then use these methods to evaluate the performance of current systems. This understanding could then be used to identify appropriate hydraulic models for the purpose of identifying parameters which influence irrigation performance in BCISs.
In developing appropriate irrigation evaluation techniques for BCISs, a variety of evaluation methods were employed on a commercially operated BCIS in the Murrumbidgee Irrigation Area (MIA) of south eastern Australia. Field measurements were taken during a number of irrigations in the 2007/08 irrigation season from a central furrow in each bay of the three bay system. It was assumed that advance across the bay would be uniform given the positive slope of each bay. Observed variation in the advance front between furrows within individual bays suggested advance was not uniform.
Consequently, several furrows were instrumented in the subsequent irrigation season of 2008/2009. Evaluation results showed a significant difference (p<0.05) between
trafficked (wheel) and non-trafficked (non-wheel) furrows for factors of furrow inflow rate, advance and furrow base elevation. On average, inflow rate into wheel furrows was 37% higher than into non-wheel furrows and wheel furrow base elevation averaged 17mm lower than non-wheel furrows, or 38% of the design furrow elevation. As a result of
this variation between furrows, a considerable negative crop response was anticipated.
However, while insufficient crop samples were collected to provide a statistically reliable analysis of within bay yield variation, field scale production yields were above the national production average suggesting any impact to be less than anticipated. It is assumed that
post-irrigation lateral redistribution of profile moisture may mitigate variability, especially in the fields of this study where an equal ratio of wheel and non-wheel furrows existed. In contrast to the measured variation within individual bays, application depths varied considerably between bays during each irrigation event. In one measured irrigation the highest application depth was 255% of the lowest applied depth. It was concluded, as
a result of this substantial variation, that the greatest potential for improving irrigation performance in BCISs was in reducing the variability in applied depths between individual bays. To reduce variability, an understanding of the design and management features that affect application depth in BCISs was required. Consequently, the potential of various hydraulic simulation models was examined.
Despite a number of hydraulic models with capacity to simulate various aspects of BCISs, none had capacity to describe irrigation at both the bay and field scales.
Consequently, a simulation model was developed to describe both within-bay irrigation and the hydraulic interaction between bays; viz the B2B model. To achieve this, a surface
irrigation hydraulic design model (Clemmens, 2007a,b) was adapted to accommodate the elements associated with a positive field slope. Parallel routines of this model where then coupled using a routine based adaptation of the Darcy-Weisbach equation to describe bay-to-bay hydraulics, thus enabling hydraulic simulation of an entire BCIS field.
B2B simulations were then used to demonstrate the capacity of the model and to test the sensitivity of BCISs to various design and management variables. Current assumptions
within the B2B model limit the model to describing general trends in Distribution Uniformity (DU). This capacity provides an important tool to examine the effect design
and management variables have on the performance of the system. Variables examined within this dissertation include bay dimensions, the vertical separation between bays,
slope, field supply rate, delivery pipe capacity, irrigation deficits and duration.
The results showed DU down the furrow to be more sensitive to adjustments in bay length than width, with performance declining as completion of advance became reliant on field
supply ‘base’ flow. As the vertical step between bays was increased, an increase in furrow inflow was apparent, commensurate with the increasing hydraulic head between the bays. However, despite the higher inflow, the impact on overall irrigation performance was relatively minor. The higher inflows generated a faster advance. However, the benefits of the higher discharge lasted for a shorter duration. This resulted in a reliance on the ‘base’ flow, similar to the above, for completion of advance which ultimately undermined the performance gains generated by the higher, but short duration inflows. Similar results
were achieved for scenarios where pipe diameter, and thus capacity were increased. B2B simulations of slope indicated that any increase in slope reduces DU in the field.
Furthermore, as slope increases, the depth of flow at the furrow inlet increases to a point where waterlogging at the inlet end of the bay is apparent. However, the presence of
some slope within the bay reduced the risk of internal drainage and also assisted in the management of irrigation water where topographical constraints limit the ‘step’ between bays. Where water ‘backs up’ into the upstream bay, the presence of a positive field slope assists in constraining water to the bankless channel. Increasing the field deficit improved the simulated DU for each bay. However, to satisfy the higher deficits irrigation duration was increased. For the infiltration characteristic
used in these simulations, a prolonger irrigation interval was required resulting in the accumulation of a considerable surface storage volume, and thus depth, in each bay. While simulations were theoretical, it was concluded that consideration must be given to water depth when increasing irrigation deficits.
The B2B model provides a design simulation capacity providing a useful resource for describing trends in irrigation performance across a BCIS field. However, the
model relies on reliable estimates of the infiltration characteristic of a field and does not simulate variation within individual bays. Consequently, evaluation of irrigation performance is required using field measurement. To effectively evaluate and determine suitable infiltration parameters for a field, this research identified several necessary field measurements as necessary: relative furrow elevation, furrow and bay inlet/outlet discharge, furrow advance and water depth at the furrow inlet. These measurements enable the infiltration characteristic for a field to be estimated and provide an insight into the uniformity of application between the bays of a field. |
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