Soil loss and degradation due to water erosion are major issues for hillside farmers. For example, in Rwanda, crop productivity in the highlands is decreasing as a result of intensive farming on steep slopes which has caused soil loss and declining soil fertility (Clay and Lewis, 1996; Kagabo et al., 2013). Soil losses in the north-western highlands of Rwanda range from 35 t ha⁻¹ yr⁻¹ to more than 100 t ha⁻¹ yr⁻¹ depending on the agricultural practices and steepness of the slope (Lewis, 1988). In Ethiopia, annual soil loss from croplands is 35 t ha⁻¹ yr⁻¹ resulting in a 1–2% annual loss in crop production (Hurni, 1993).

In general, terracing conserves soils compared to non-terraced fields regardless of the cultivation system used to produce field crops. Nevertheless, soil erosion and the loss of topsoil are still major threats to terrace farming. Terracing affects the rate of soil erosion caused by water through its effect on local hydrology, runoff characteristics, soil moisture and soil characteristics (Chow et al., 1999). It is obvious that the effective utilization of terrace lands and maintenance of terrace walls can reduce runoff and soil losses (AAFC, 1999) but terracing also disturbs the soil strata, and considerable soil loss occurs during construction and in the first few years, leading to initial declines in soil fertility (ICIMOD, 1998).

Soil erosion control by terracing is often found to be the most expensive soil conservation practice (Inbar and Llerena, 2000) as it requires tremendous labor and investment for construction and maintenance of the terrace walls. As a result, terrace abandonment and terrace deterioration are observed more often in areas with local labor shortages, which result in massive soil losses (Vogel, 1988; Cerda-Bolinches, 1994; Harden, 1996). Gallart et al. (1994) explained that terraces retain an excess of water leading to saturation, and consequently storm runoff can affect the base of terrace walls due to steepness and sparse vegetation cover (Lasanta et al., 2001; Van Dijk and Bruijnzeel, 2003). Saturation and storm runoff lead to further deterioration of terraces due to gully formation. In Tanzania, bench terracing was found to be inappropriate in areas having thin topsoil as it exposed the infertile subsoil during construction, held excess water, and triggered landslides (Temple, 1972). Soil loss from bench terraces was ~5 t ha⁻¹ yr⁻¹ under rainfed conditions (Carson, 1992). Terracing increases soil loss if constructed in sandy and coarse textured soils and on very steep slopes (ICIMOD, 1998).

Changes in soil characteristics after terracing degrade soil quality (Hamdan et al., 2000; Li and Lindstrom, 2001) through increased runoff and soil erosion (Ternan et al., 1996). Even within the terrace, soil fertility increases in the lower part of terraces compared to the upper part due to the down slope movement of organic matter and nutrients (Gebremedhin et al., 1999; Walle and Sims, 1999; Dercon et al., 2003; Kagabo et al., 2013). For this reason, it is considered efficient to initially construct small contour ridges made of vegetation and stones compared to the diversion terraces, to entrap sediments and protect soil strata, permitting gradual terrace formation after 4–10 years (Roose, 1986).

Table 2 shows examples of low-cost practices and tools that provide opportunities to intensify terrace cropping systems while improving sustainability and/or drudgery. Terraces offer opportunities to offset agricultural losses related to low and erratic rainfalls in hills and mountains by utilizing the inverse slopes and by adopting soil moisture conservation tillage technologies (e.g., contour ridging, tied-ridging, and mulch-ripping on hillsides or by adopting zero or minimum tillage) in order to increase germination and yields (Vogel et al., 1994; Guto et al., 2012; Chen et al., 2015). In Zimbabwe, no till tied-ridging and ripping into maize residues greatly reduced surface runoff and increased the infiltration rate, resulting in higher grain, and biomass yields due to increased root depth and root length density (Vogel et al., 1994). Ridges and tied-ridges can be constructed using local equipment (e.g., mouldboard plow) that is designed to be animal-drawn. Similarly, the use of plastic film combined with straw mulch in winter wheat increased grain yield (35%) and water use efficiency (25%) compared with conventional practices in the Loess Plateau, China (Chen et al., 2015). The combination of minimum tillage and the living vegetative barriers of the leucaena tree (Leucaena trichandra Zucc. Urb.) also resulted in reduced competition for water between barriers and companion crops in the water deficient highlands of Kenya (Guto et al., 2012). In this region, yields of maize and soybean were shown to be suppressed by the barrier-crop interface (e.g., due to shading) but the yield losses were consistently compensated by improved crop performance at the center of the terraces. Similarly, in the Anjenie watershed of Ethiopia, terrace farming showed increased yields of maize (1.73 t ha⁻¹) and barley (1.86 t ha⁻¹) over the control (0.77 and 0.61 t ha⁻¹ for maize and barley, respectively) as a result of water conservation and erosion control (Adgo et al., 2013), resulting in improved household income and food security.

In high hills and mountains, crops require a longer growing season than low altitudes due to cooler temperatures. Transplanting of vegetative parts (cuttings, tubers, rhizomes) or seedlings from nurseries (e.g., grown in plastic greenhouses) may mitigate this challenge. Plastic greenhouses (i.e., semi-circular to square shaped high tunnels) may also be used to introduce certain high value crops [such as tomato, cucumber, runner bean (Phaseolus coccineus L), etc.] by replacing or adding to less profitable field crops. Intercropping (i.e., growing of two or more crops together on the same land) is another opportunity to harvest multiple crops in the same season, increasing land productivity (Chapagain and Riseman, 2012, 2014a,b; Chapagain, 2014) and other ecosystem functions (e.g., nutrient cycling, carbon sequestration, water use efficiency, etc.) in smallholder agriculture (Chapagain and Riseman, 2015; Chapagain, 2016; Thilakarathna et al., 2016). Relay intercropping, where the second crop is seeded after the first crop has reached its reproductive stage but prior to harvesting, also takes advantage of a shorter available growing season. For example, planting of millet, soybean, horsegram, and runner beans before maize is ready for harvest is common in the hills of Nepal (Sharma et al., 2001).

Tools listed in Table 2 are also available on a commercial scale, and they can be procured in Asia at a large scale online such as from Alibaba.com, Indiamart.com, etc. Tools for land preparation (e.g., mini-tillers), sowing (e.g., jab drill planters), weeding (fork and cono weeders), harvesting (corn shellers, millet thresher, etc.), and protective equipment (e.g., knee-pads, gloves) may be effective in reducing drudgery and discomfort, especially for women farmers. Most of the listed tools come at a price ranging from $1–10 which can be purchased by an individual farmer or household; however, a few big machines (such as mini-tillers, electric maize and millet threshers, etc.) may cost up to $500 which can be purchased as a communal-tool by a farmer's group or cooperative and/or at a subsidized price if provisioned by the national government as seen in Nepal (SAKNepal, 2017).

As a wide variety of crops ranging from small herbs to large trees can be grown on terraces, there may be opportunities to diversify and intensify terrace agriculture. The selection of crop and cropping systems is dependent on farmers' decisions, which are conditioned by multiple drivers such as climate, soil type(s), topography, land holdings, farmers' needs, cultural preferences, availability of agricultural inputs (e.g., seeds, fertilizers, etc.), and local market opportunities (Riley et al., 1990; Chapagain and Good, 2015).

In Nepal, for example, the principal field crops grown on terraces include maize (Zea mays L.), rice (Oryza sativa L.), and finger millet (Eleusine coracana L.), while crops such as wheat (Triticum aestivum L.), common bean (Phaseolus vulgaris L.), field pea (Pisum sativum L.), and underutilized and wild legumes can also be planted depending on the season and farmers' interest (Riley et al., 1990; Wymann von Dach et al., 2013). In addition, several vegetables are also grown in terraces including potato (Solanum tuberosum L.), tomato (Solanum lycopersicum L.), cucumber (Cucumis sativus L.), eggplant (Solanum melongena L.), okra (Abelmoschus esculentus L.), chile (Capsicum annuum L.), bitter gourd (Momordica charantia L.), and spices such as ginger (Zingiber officinale Roscoe), turmeric (Curcuma longa L.), onion (Allium cepa L.), garlic (Allium sativum L.), and other minor crops. Furthermore, terraces in hills of Nepal and other South Asian countries are sources of a wide variety of medicinal herbs (Table 4, organized from herbs to trees) that reportedly offer health benefits.

The Taino were a pre-Columbian farmer society in the Caribbean who developed a sustainable system of hillside agriculture by raising their crops in conucos, large mounds created on slopes containing complex intercrops including root crops such as squash, sweet potatoes, and yams, along with maize and other New World crops (Watts, 1987). The conuco system employed principles of conservation farming including: ensuring the ground was never left bare in part through the use of perennial intercrops such as cassava; use of twigs/mulches to intercept rainwater; and intercropping with nitrogen fixing legumes such as common bean and peanuts. These strategies apparently permitted some mounds to be productive for up to 20 years. Farmers first set fire to the brush before planting root crops to create more fertile soil. The women then used a type of hoe called a coa to transplant cuttings into the earth. This system of shifting cultivation was very well suited to the Caribbean environment as it provided good drainage and reduced erosion (Watts, 1987). Though marginalized, conuco farming is still in practice today in the Caribbean mountains, especially in Haiti and the Dominican Republic (Houston, 2005).

Rice terraces can be integrated with fish farming to optimize resource utilization through the complementary use of water and land (Frei and Becker, 2005). This system uses conventional flooded water management practices to increase productivity, profitability and sustainability (Ahmed and Garnett, 2011). The fish improve soil fertility by increasing the availability of oxygen (aeration) and by depositing nitrogen and phosphorus (Giap et al., 2005; Dugan et al., 2006). Furthermore, farmers employ this method for biological control of rice pests (flies, snails, and other insects), and hence the rice-fish system is regarded as an important element of integrated pest management (IPM) in rice (Berg, 2001; Halwart and Gupta, 2004). Fish act as predators, and help control aquatic weeds and algae that act as hosts for pests and compete with rice for nutrients. Moreover, fish eat the eggs and larvae of disease causing insects (e.g., malaria causing mosquitoes, etc.) and help control water-borne diseases (Matteson, 2000). In turn, rice provides fish with planktonic, periphytic, and benthic food (Mustow, 2002). The water temperature is also maintained by the shading effect of the rice, enabling fish to thrive during hot summer months (Kunda et al., 2008).

Azolla is a highly productive and free floating aquatic fern that fixes atmospheric nitrogen is association with the nitrogen-fixing cyanobiont, Anabaena azollae. Azolla is able to double its biomass in 2–3 days (Kannaiyan, 1993) and is used as an organic bio-fertilizer in rice fields in Asia, but is much less common in East Africa where there is an opportunity to expand the practice. Temperature and light are the most important factors that influence the growth and efficiency of nitrogen fixation in the Azolla-Anabaena symbiosis in the tropics (Becking, 1979). Therefore, selection for temperature tolerant and photo-insensitive strains of Azolla (e.g., A. microphylla) represent opportunities (Kannaiyan and Somporn, 1988). Inoculation into transplanted rice fields with the fresh biomass of Azolla fronds (200 kg ha⁻¹) or the frond based spore inoculum of A. microphylla (2.5 kg ha⁻¹) can produce ~15–25 tons of a fairly thick layer of Azolla (Kannaiyan and Somporn, 1987; Kannaiyan, 1993). The symbiosis adds nitrogen (40–60 kg ha⁻¹) and organic matter to the soil after decomposition, and has been shown to cause a 36–38% higher grain yield compared to a sole rice system (Kannaiyan, 1993). Apart from Azolla, use of alternate wetting and drying (AWD) or intermittent irrigation helps improve crop performance, productivity and water-efficient production of rice over conventional flooding in water deficit areas (Chapagain and Yamaji, 2010; Chapagain et al., 2011a,b).

Hills and mountains possess diverse climatic conditions that permit farmers to grow a variety of agronomic and horticultural crops. In Nepal, for example, there are four different agro-ecological zones, which can be exploited to produce off-season vegetables, fruits, and other cash crops throughout the year (Panth and Gautam, 1990). The presence of niche based micro-climatic pockets within each of these agro-climatic regions further provides tremendous opportunities to grow a diversity of food crop, fibers, fruit, medicinal plants, and fodder trees of economic value to that region.

Natural slopes on hills and mountains offer opportunities to take advantage of gravity for creative water capture, irrigation, and livestock urine collection. Besides the construction of tied-ridges, rips, and use of inverse slopes during the dry season, gravity can be utilized to irrigate field crops and to capture urine from penned livestock, and then send the water by PVC pipe or locally-sourced bamboo to FYM/compost pits below, to enrich the nutrient content. In areas equipped with drip irrigation structures, plants can be irrigated with urinated water at no additional cost.

Land topography (e.g., east or west facing slopes to different degrees) in hills and mountains further provides opportunities to produce high value crops on slopes and terraces based on their light and moisture requirements. The direction that a slope faces determines when crops are exposed to sunshine during the day. For example, slopes facing northeast in Nepal have successful citrus cultivation due to the availability of early morning sunshine followed by shade at noon that helps conserve soil moisture, whereas plots facing southwest at the same elevation are devoid of citrus trees (Shrestha et al., 2001). Since soil types differ with the land topography, crops that require different soil, climate, and topography conditions can be produced on hills and terraces. In addition, this situation creates an opportunity to adopt site-specific agroforestry systems (crops, trees, pastures, and livestock together).

In addition to permanent migration (noted above), in the high hills and mountains of developing countries, there is significant seasonal outmigration of farmers during the dry season following harvesting of the main crops (Patel et al., 2015; Gartaula et al., 2016). Farmers migrate to nearby cities and towns for alternative income opportunities such as from carpentry, house/road/bridge construction, etc. This situation can be minimized by introducing practices that utilize the fallow land for planting forages, along with planting of high value crops (seed, vegetable, cash) combined with water harvesting in the rainy season and drip irrigation (SAKNepal, 2017). Selection of crops and/or varieties with different root architectures (i.e., longer and finer roots, including greater number of tips and branching angle, and a lower shoot:root ratio, Chapagain et al., 2014) and in situ moisture conservation practices (ridging, mulching, Watts, 1987) may further help to minimize irrigation requirements during dry periods.

Hillside terraces promote eco-tourism. Rice terraces in the Philippines, China, and Japan are very good examples where communities gain income from eco-tourism. The Rice Terraces of the Philippine Cordilleras and the Hani Rice Terraces in Yuanyang, China, were inscribed on the UNESCO World Heritage List in 1995 and 2013, respectively. The Cordillera terraces were the first-ever property to be included in the cultural landscape category of the World Heritage List which helped to increase the number of tourists and income from tourism. The Hani Terraces were built on mountain slopes ranging from 15 to 75° and provide a typical example of the harmony between people and nature; tourists visit to learn about and photograph the local rice farming and ethnic cultures (Lu, 2015).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.