Field-scale assessment of deep drainage risk

Improving irrigation efficiency is of primary importance in arid and semi-arid regions of the world as a consequence of increasing incidences of soil and water salinisation. In the cotton-growing regions of Australia salinisation is generally a result of inefficient irrigation practices, which lead to excessive deep drainage (DD). There is therefore the need to apply a relatively inexpensive approach to assessing where inefficiencies occur and make prediction of suitability of existing and new water storage sites. However, physical methods of measuring DD, such as flux meters and lysimeters, are time-consuming and site-specific. In this paper we apply a rapid method for determining the spatial distribution of soil in an irrigated cotton field in the lower Gwydir valley. First, EC_a data (using EM38 and EM31) were used to determine a soil-sampling scheme for determining soil information such as clay content and exchangeable cations to a depth of 1.2 m. The soil data and water quality information were input into the SaLF (salt and leaching fraction) model to estimate DD rate (mm/year). In developing the relationship between EC_a and estimated DD, three exponential models (two-, three- and four-parameter) were compared and evaluated using the Aikakie information criteria (AIC). The three-parameter exponential model was found to be best and was used for further analysis. Using the geostatistical approach of multiple indicator kriging (MIK), maps of conditional probability of DD exceeding a critical cut-off value (i.e. 50, 75, 100 mm) were produced for various rates of irrigation (I=300, 600, 1,200 and 1,500 mm/year). The areas of highest risk were consistent with where water-use efficiency was problematic and thus leading to the creation of perched water tables. The advantage of this approach is that it is quick and is applicable to situations where efficient use of water is required. The results can be used for irrigation planning, particularly in the location of large irrigation infrastructure such as water reservoirs.     

The Viejo Retamo secondary canal

The intended operation of the Rio Tunuyan irrigation scheme in the province of Mendoza, Argentina, is to divide flow proportionately over the entire area in accordance with registered water-rights. The main division of flow is by regulators at the heads of the lateral canals, which serve areas of appr. 15,000 ha. Taking the sub-lateral or secondary canal Viejo Retamo as an example, this paper discusses the flow division downstream of these regulators. The operation of the canal is evaluated with two performance indicators:

-the ratio of volume of water intended to be supplied to the tertiary units over the volume of water actually supplied, and

-the misallocation of water in volume.

In the Rio Tunuyan scheme, there is also a relation between the evapotranspiration of crops and the groundwater level: if less than 60% of the supplied canal water evapotranspires, the groundwater level rises.     

Interfacial contribution to two-electrode soil moisture sensor readings

Summary Contribution of electrochemical polarization, occurring at the electrode-moist soil interface, to soil moisture sensor readings is discussed for two bare electrodes or for soil moisture blocks. A soil moisture sensor can be considered as an electrolytic cell, the electrical impedance of which is the resultant of several component subimpedances arising from a diversity of phenomena causing sensor electrical polarization. It is shown that within the range of commonly applied frequencies: readings of sensors' electrical capacitance are totally masked by interfacial pseudocapacitance while the readings of the sensor electrical resistance are influenced by interfacial phenomena unless the read-out device compensates for the capacitive component of the sensor impedance.     

An integrated modelling toolbox for water resources assessment and management in highland catchments: Model description

Water and land resource competition and environmental degradation pose difficult questions for resource managers. In particular, the ensuing trade-offs between economic, environmental, and social factors and their spatiotemporal variability must be considered when implementing management policies. This paper describes an integrated modelling toolbox that has been developed for highland catchments – specifically the Mae Chaem catchment in Northern Thailand. This toolbox contains models of crop growth, erosion and rainfall-runoff, as well as household decision and socioeconomic impact models. The approach described advances and complements previous approaches by: considering more complex interactions between land-use decisions and the hydrological cycle; modelling household decisions based on uncertain expectations; and assessing impacts of changes not only on flows and household income, but also on subsistence production and erosion. An example of the types of trade-offs and scenarios that can be assessed using the integrated modelling toolbox is also presented. This demonstrates that for the scenarios presented, the magnitude and direction of impacts simulated by the model is not dependent on climate. Further testing of the model is demonstrated in a companion paper. Overall, the plausibility of the model is shown.     

Effects of soil type on soybean crop water use in weighing lysimeters

Summary Different soils are known to affect the amount and distribution of both available water and roots. Optimising irrigation water use, especially when shallow water-tables are present requires accurate knowledge of the root zone dynamics. This study was conducted to determine the effect of two soil types on root growth, soil water extraction patterns, and contributions of a water-table to crop evaporation (E). Two weighing lysimeters (L1 and L2) with undisturbed blocks of soil were used. The soil in L1 had higher hydraulic conductivity and lower bulk density than that in L2. Well watered conditions were maintained by irrigation for the first 110 days from sowing (DFS). Root length density (RLD) was calculated from observations made in clear acrylic tubes installed into the sides of the lysimeters. Volumetric soil water contents were measured with a neutron probe. A water-table (EC = 0.01 S m^-1) was established 1 m below the soil surface 18 DFS. RLD values were greater in L1 than L2 at any depth. In L1, maximum RLD values (3 × 10⁴ m m^-3) were measured immediately above the water-table at physiological maturity (133 DFS). In L2, maximum RLD values (1.5 × 10⁴ m m^-3) were measured at 0.42 m on 120 DFS and few roots were present above the water-table. From 71 to 74 DFS, 55 and 64% of E was extracted from above 0.2 m for L1 and L2, respectively. In L2, extraction was essentially limited to the upper 0.4 m, while L1 extraction was to 0.8 m depth. Around 100 DFS the water-table contributed 29% (L1) and 7% (L2) of the water evaporated. This proportion increased rapidly as the upper soil layers dried following the last substantial irrigation 106 DFS. Over the whole season the water-table contributed 24% in L1 and 6.5% in L2 of total E.     

Water balance estimates of evaporation from ponded rice fields in a semi-arid region

Evaporation from ponded rice grown on fine-textured soils was calculated using a simple water balance equation, and compared with pan evaporation measured at a regional meteorological station. Crop/pan (E_r/E_p) evaporation ratios showed seasonal trends, with values generally exceeding unity during the mid- to late-season ponding period. Mid-summer ratios of E_r/E_p for two experimental fields were not significantly different from those reported in a previous study using a different experimental technique, but were higher on a third. On the average, evaporation over the whole ponding period equalled that from a Class A pan. The assumption that the infiltration or seepage term of the water balance may be neglected for fine-textured soils would seriously overestimate E_r on one of the three experimental sites.     

Dihydrooxadiazines: Octopaminergic system as a potential site of insecticidal action

Dihydrooxadiazines are structural analogs of octopamine and were compared with octopamine for their ability to compete with [³H]dihydroergocryptine ([³H]DHE) for binding sites on DHE-sensitive receptors, to stimulate adenylate cyclase activity in nervous system homogenates of Periplaneta americana L., and to modulate the action of the peptide proctolin on the oviducal muscles of Locusta migratoria L. [³H]DHE binding was inhibited by low concentrations (μM range) of octopamine, phentolamine, N-demethylchlordimeform (DCDM) and several dihydrooxadiazines. The tested dihydrooxadiazines acted as aminergic agonists in stimulating cyclic AMP production in cockroach nervous system homogenates and did not show additive effects with octopamine, whereas additivity was observed with 5-hydroxytryptamine. The relative potency of octopaminergic antagonists, including mianserin, cyproheptadine, phentolamine, and gramine, to block octopamine-mediated elevation of cyclic AMP production was similar to the rank-order potency of the same antagonists to inhibit dihydrooxadiazine-mediated elevation of cyclic AMP production. Octopamine, 2-(4-bromophenyl)-5,6-dihydro-4H-1,3,4-oxadiazine (4-Br-PDHO), and 8-Br-cyclic AMP caused increased phosphorylation of proteins that are phosphorylated by exogenously added cyclic AMP-dependent protein kinase. These results indicate that the dihydrooxadiazine-induced rise in cyclic AMP levels in homogenates of the cockroach nervous system results directly in activation of an endogenous cyclic AMP-dependent protein kinase. 4-Br-PDHO behaved similarly to octopamine in modulating the action of proctolin-induced contractions in locust oviducal muscles. These observations suggest that dihydrooxadiazines act as octopamine agonists and have an octopaminergic action in modulating the action of proctolin. Thus, it is proposed that dihydrooxadiazines exert at least part of their insecticidal and miticidal actions through interaction with the octopaminergic system.