Effective Rainfall Estimation Methods

One common problem is associated with water balance calculation methods for determining soil moisture for scheduling irrigation: errors in the estimated soil moisture are cumulative and frequent recalibrations are needed. A simple and robust approach to estimation of daily soil moisture using a daily diagnostic soil moisture equation is suggested and studied. The estimated soil moisture is a function of the time-weighted summation of the ratio of historical precipitation rate to soil moisture loss coefficient. To capture the seasonal variation in soil moisture loss coefficient, a sinusoidal wave function of the day of year (DOY) is used to represent the seasonal variation in loss coefficient. A 3-year continuous data set of daily soil moisture and daily precipitation collected at each of four Soil Climate Analysis Network sites-AR2091; in Arkansas, GA2013 in Georgia, NM2107 in New Mexico, and PR2052 in Puerto Rico-is applied to test the proposed method. The land cover/land use of these four sites is agricultural/crop fields, grasslands, or desert. Root mean square errors of the estimated volumetric soil moisture are less than 5% (v/v), and all correlation coefficients, R 2 , are greater than 0.78. The results indicate that there are three advantages associated with the suggested approach: (1) errors in estimated soil moisture are noncumulative; (2) regular recalibration is not required; and (3) numerical iteration and initial moisture information are not required.

International Journal of Climatology, 2009

Soil water is an important parameter in the hydrological balance and is essential for understanding land-surface interactions. While spatial coverage of soil water observations is poor across most of the United States, impedance probes have been used to measure soil water throughout Nebraska in the Automated Weather Data Network (AWDN) since 1999. With 8 years of continuous data at selected sites, we were able to derive a Soil Moisture Index (SMI) that is based on an actual water content (θ ) and a known field capacity and wilting point. The SMI is a continuous function and is scaled from 5.0 to −5.0, with 5.0 representing θ at field capacity and −5.0 representing θ at wilting point. Since the inception of the soil water network in 1999, periodic episodes of drought have affected all areas of Nebraska. Thus, the SMI was used in this study to analyse growing seasons (1 April to 1 October) from 1999 to 2006 at six selected sites throughout Nebraska. The SMI proved to be effective at depicting the onset of drought and at identifying soil recharge, or lack thereof, when heavy rainfall events followed a significant dry spell.

A soil moisture accounting (SMA) algorithm was developed by the Hydrologic Engineering Center for the Hydrologic Modeling System (HEC-HMS). HEC-HMS succeeds the widely used HEC-1 hydrologic model as part of the next generation (NEXGEN) of hydraulic and hydrologic modeling software under development at the U.S. Army Corps of Engineers Hydrologic Engineer Center (HEC). As originally released in 1998, HEC-HMS was an event based simulation model with the exception of a simple deficit/constant loss function. SMA was implemented as a loss rate method in the HMS version released in the year 2000 (Version 2.0) and allows for long-term continuous simulation of hydrologic processes that occur and change over time in a watershed. This is achieved by simulating the movement of precipitation through storage volumes that represent canopy interception, surface depressions, the soil profile and two groundwater layers. Computational components of this algorithm also include evapotranspiration (ET), surface runoff, and groundwater flow. This paper details the computational steps and equations used in the SMA loss method to simulate these processes. Results of model application to simulate runoff for two watersheds are described. The watersheds are near Tifton, Georgia, and Riesel, Texas, and periods of simulation were eleven and seven years, respectively.

Agricultural Water Management, 2010

The amount of soil water at the beginning of the growing season has a large impact on crop yields in rainfed agriculture, especially in semi-arid regions and in years with below-average rainfall in more humid climates. Robust algorithms are needed to estimate soil water storage before planting to aid crop management decisions. The main objectives of this paper are to investigate soil water recharge during the non-growing season (October 20 to May 1) in a semi-arid, temperate ecosystem in southcentral Nebraska (USA) and to evaluate empirical models to estimate soil water content at the beginning of the summer-crop growing season. A database of soil water content measurements collected over 5 years at nine locations in south-central Nebraska was used to estimate available water-holding limits in the soil profile and to determine the change in available soil water during the non-growing season.

Hydrological Processes, 2004

A criterion is developed for determining the validity of the Soil Conservation Service curve number (SCS-CN) method. According to this criterion, the existing SCS-CN method is found to be applicable when the potential maximum retention, S, is less than or equal to twice the total rainfall amount. The criterion is tested using published data of two watersheds. Separating the steady infiltration from capillary infiltration, the method is extended for predicting infiltration and rainfall-excess rates. The extended SCS-CN method is tested using 55 sets of laboratory infiltration data on soils varying from Plainfield sand to Yolo light clay, and the computed and observed infiltration and rainfall-excess rates are found to be in good agreement. Copyright © 2004 John Wiley & Sons, Ltd.

Transactions of the ASABE

HighlightsSWAT soil water assessment was performed using soil water measurements.Dryland SWAT model soil water content was greater than the irrigated SWAT model.Using SWAT soil water estimates for real-time (daily) irrigation management purposes with the existing SWAT soil water subroutines and available soils data is considered risky.The surface layer showed the greatest soil water variability compared to deeper layers.. Soil water content (SWC) is a challenging measurement at the field, watershed, and regional scales. Soil and Water Assessment Tool (SWAT) soil water estimates were evaluated at three locations: the St. Joseph River watershed (SJRW) in northeast Indiana, the USDA-ARS Conservation and Production Research Laboratory (CPRL) at Bushland, Texas, and the USDA-ARS Limited Irrigation Research Farm (LIFR) at Greeley, Colorado. The soil water estimates were evaluated under two scenarios: (1) for the defined soil profile, and (2) by individual layer. Each site’s soil water ass...

Evapotranspiration is an important component of the hydrologic cycle. In this research, an attempt has been made to estimate the potential and actual evapotranspiration combining different approaches that can be easily adapted in many of the irrigation projects found in the Tigray region of Ethiopia. There is no established weather station in these project areas. Therefore, it was not possible to use more complicated equations like the Penman family to estimate the potential evapotranspiration that can be used during planning and operation of irrigation projects. Thus, as an alternative in this research, an empirical equation that uses less data input has been adapted and then calibrated with the estimated values from the Modified Penman equation.

Land, 2023

In the Czech Republic, the Universal Soil Loss Equation provides the basis for defining the soil protection strategy. Field rainfall simulators were used to define the actual cover-management factor values of the most extensively seeded crops in the Czech Republic. The second purpose was to assess rainfall-runoff ratio for different crops and management to contribute to the debate of water retention effectiveness during approaching climate change. The methodology focused on multi-seasonal measurements to cover the most important phenological phases. The rainfall intensity was 60 mm·h−1 for 30 min and a plot size of 16 m2. More than 380 rainfall simulation experiments provided data. Soil conservation techniques proved to have a significant effect on runoff reduction. Conventionally seeded maize can reduce the runoff ratio to around 50%. However, cover crops combined with reduced tillage or direct seeding can reduce the runoff ratio to 10–20% for ‘dry’ conditions and to 12–40% for ‘saturated’ conditions. Conventionally seeded maize on average loses 4.3 Mg·ha−1 per 30 min experiment. However, reduced tillage and direct seeding reduce soil loss to 0.6 and 0.16 Mg·ha−1, respectively. A comparison with the original USDA values for maize showed that it is desirable to redefine the crop cover factor.

Journal of Hydrology, 2006

The focus of this paper is to develop a methodology to answer the question: do the spatially distributed soil water estimates produced by operational distributed hydrologic models provide potential benefits for agriculture? The formulation quantifies the potential value through a cost-loss analysis, whereby cost for the farmer is associated with the decision to irrigate the field and loss is associated with the decision not to irrigate while damaging soil water deficits occur. Farmer decisions are made in view of the likelihood of damaging events as estimated by the ensemble distributed model simulations of soil water deficit. The ensemble simulations account for parametric and radar rainfall uncertainty. The application area for the economic value analysis is the farmland of the Illinois River watershed in northwestern Arkansas (mainly) and eastern Oklahoma, for which operational-quality distributed model input is available. The land is used to produce hay for feed. The analysis indicates that there is substantial potential economic value in using the ensemble soil water estimates to make decisions regarding irrigation within the watershed for the months of July, August and September, when severe soil water deficits may occur. The benefits are higher for lower cost-loss ratios and for higher yield plants. They exhibit considerable spatial variability within the watershed in agreement with the spatial variability of the incidence of soil water deficits and with the spatial variability of the ability of the ensemble model simulations to reproduce this variability. The results of this study warrant additional analysis of the economic value of distributed model simulations in other regions, different distributed models and for other types of crops. Consideration of forecasts in addition to simulations is also an important next step.

Journal of the American Water Resources Association, 1986

A comparison of 13 different methods of estimating mean areal rainfall was made on two areas in New Mexico, U.S.A., and one area in Great Britain. Daily, monthly and yearly rainfall data were utilized. All methods, in general, yielded comparable estimates, especially for yearly values. This suggested that a simpler method would be preferable for estimating mean areal rainfall in these areas.